IXL A.1 Grade 9

Emergency! Prior to 1968, the United States did not have a nationwide phone number for emergency assistance; instead, emergency numbers varied from region to region. When people required urgent help from the police or the fire department, they had to either call the operator or dial their local services directly. Concerned about the dangerous inefficiency of this system, the federal government decided to establish a single emergency number that could be used throughout the country. The telecom company AT&T proposed that the code be 911, a sequence that was easily remembered and not already in use as an area code or service code. This proposal was accepted, and the first 911 call was made by Alabama Senator Rankin Fite on February 16, 1968.

To replace an inefficient emergency call system, the U.S. government instituted nationwide 911 service.

Many clinicians have found that the treatment can be effective for certain types of severe wounds that do not respond to antibiotics.

It establishes the modern-day relevance of maggot therapy.

Berlin Wall Tunnel After Germany was defeated in World War II, it was divided into two sectors. The western part became West Germany, and the eastern part became the Soviet-controlled German Democratic Republic (GDR), or East Germany. Although Berlin was entirely inside East Germany, the city was also divided, with East Berlin becoming the capital of East Germany and West Berlin becoming part of West Germany. In 1961, East Germany erected a wall between East and West Berlin to prevent residents from defecting, or escaping, from East Germany's oppressive communist regime. Concerned about family and friends stuck in East Berlin, a group of West Berlin students came up with a plan. After setting up camp in an abandoned bakery, they spent five months excavating a secret tunnel that wound its way under the wall to an East Berlin apartment building. The students instructed their loved ones in East Berlin to come to the apartment building and gain entrance with a special code word. The rescuers crawled through the tunnel, met the escapees, and guided them to freedom.

In order to rescue loved ones trapped in East Berlin, a group of West Berliners dug an underground passage.

Camera-Like Memory Do some individuals have the amazing ability to take perfectly accurate, permanent mental "snapshots," which can be stored in the brain for future retrieval? Fictional figures aside, no. But while this popular notion of "photographic memory" is unfounded, research has demonstrated the existence of a fascinating phenomenon called eidetic memory, in which memory actually does have a camera-like quality. People with eidetic memory are able to look at an object or picture and then look away or close their eyes and continue to "see" the object or picture in their minds. These images last only a few minutes at most before dissipating, and they are typically not entirely accurate. Still, the degree of detail that "eidetikers" can recall during those first few minutes is astonishing. Interestingly, the phenomenon occurs almost entirely in children, with an estimated two to ten percent of children affected; virtually all eidetikers lose their unusual ability before reaching adulthood.

Individuals with eidetic memory are temporarily able to retain detailed images in their minds.

Franklin's work helped Watson and fellow researcher Francis Crick discover the structure of DNA, but she received no credit when they won a Nobel Prize for their model of DNA in 1962.

It describes an incident when a female scientist did not receive credit for her work.

In addition, scientists have found evidence that fevers may increase the number of certain lymphocytes—white blood cells that are part of the immune system.

It describes one manner in which fevers fight infections.

One day he noticed that mold had accidentally been introduced into one of the petri dishes and was startled to see that there were no bacteria around the moldy area.

It indicates the unintentional nature of Fleming's discovery.

Hospitals in poor countries often cannot afford the equipment, and the hospitals that do have incubators are typically located only in urban areas.

It indicates why incubators aren't a practical solution for all preemies.

Keeping Preemies Warm Premature babies—born three or more weeks short of a full-term pregnancy—often lack vital body fat, putting them at risk of dangerously low body temperatures. Such "preemies" can be warmed in hospital incubators. However, incubators are not always a viable solution in lower-income countries. Hospitals in poor countries often cannot afford the equipment, and the hospitals that do have incubators are typically located only in urban areas. Many rural families are unable to transport their preemies such long distances, and those who make the trip can generally remain at the hospital for only a few days. A group of students at Stanford University developed a solution: an inexpensive, portable baby warmer for home-based use. Similar to a small sleeping bag, the warmer contains a heating pouch which, once warmed, maintains a temperature of 98.6°F for four hours. Thanks to this innovation, thousands of preemies have been saved.

Stanford students developed a baby warmer for use in the home, saving the lives of premature babies whose families lack access to incubators.

The Bolshevik-Menshevik Divide In the early 1900s, the Russian Social Democratic Labour Party, unhappy with the poverty and war that plagued Russia, sought to rid the country of the czar, its absolute ruler. Unable to agree on tactics, the party split into two groups. The Bolsheviks, led by Vladimir Lenin, believed that Russia should be governed by a small group that would have complete power and would not tolerate dissent from its members. They favored immediate reform, and they viewed violence as an acceptable means of achieving this. In contrast, the Mensheviks, led by Julius Martov, felt that Russia should have a larger, more democratic government that would tolerate varying opinions. Unlike the Bolsheviks, they favored a gradual transition of power, and they preferred to avoid violent means of gaining control. Eventually, the Bolsheviks were victorious, seizing control in the October Revolution of 1917.

The Bolsheviks wanted a small governing group with complete and immediate control, while the Mensheviks wanted a larger, more democratic government.

Overlooked Women in Science Historically, men have dominated the field of science, and many women have not received professional credit or public recognition for their work. Rosalind Franklin, for example, was a chemist who did groundbreaking research into the structure of DNA, a type of molecule in cells that helps control their growth, activity, and reproduction. Franklin's X-ray photograph of DNA was shared, without her knowledge, with biologist James Watson. Franklin's work helped Watson and fellow researcher Francis Crick discover the structure of DNA, but she received no credit when they won a Nobel Prize for their model of DNA in 1962. Another scientist denied credit for her work was Jocelyn Bell Burnell. Bell Burnell was a postgraduate student in 1967 when she discovered the first radio pulsars—small, dense rotating stars that emit radio waves. However, the credit was given to her supervisor, Antony Hewish, who received a Nobel Prize in 1974.

Women have often failed to receive the credit they deserve for their scientific work.

Fearing for their lives, some took refuge in their basements or fled their homes.

It establishes that some people truly believed that the performance was real.

Deaf President Now When Jerry Lee, the president of Gallaudet University, resigned in 1987, a months-long search for his replacement ensued. Gallaudet, a university for deaf and hard-of-hearing students, had never been led by a deaf president, and many students hoped that a deaf candidate would be selected. They argued that a deaf president would be better equipped than a hearing president to understand and represent the Gallaudet community. Despite these concerns, the board of trustees chose a hearing president. Angered by the decision, students began a large-scale protest that became known as the Deaf President Now movement. Demanding that the newly appointed president resign and that a deaf president be appointed, students organized rallies, made speeches, wrote letters, and engaged media outlets. The protests were effective. The hearing president resigned, and on March 13, 1988, in an about-face, the board announced the appointment of a deaf president, Dr. I. King Jordan.

Due to the considerable efforts of the Deaf President Now movement, Gallaudet University appointed its first deaf president in 1988.

With a technological edge, they worried, the Soviets could dominate the exploration of space or develop superior weapons.

It details the motivation for creating NASA.

Like Rogers, Coone and Morrisett were dissatisfied with the options available for young children.

It compares the motivation of the creators of Sesame Street with that of Fred Rogers.

When people required urgent help from the police or the fire department, they had to either call the operator or dial their local services directly.

It provides background information about the old emergency system.

Saving the Manatee The Florida manatee, a subspecies of the West Indian manatee, is a large ocean mammal found in Florida's rivers and coastal areas. Unfortunately, the state's population of manatees has dwindled significantly over the years. Although the manatee has no natural predators, humans have contributed to many manatee deaths, mainly through boat collisions. The loss of warm-water habitats has also hastened the manatee's decline. Aerial surveys performed in the early 1990s found an estimated 1,267 manatees in Florida. Because of conservation efforts by several organizations, however, the number of manatees has rebounded recently. The creation of manatee protection zones—areas that boaters must avoid or drive through more slowly—has been particularly effective. As a result of these efforts, more than 6,000 manatees are found in Florida today.

Thanks to conservation efforts, Florida manatee populations have increased.

Though proven effective in flu prevention, the influenza vaccine, or "flu shot," won't keep you from contracting the stomach flu.

It addresses a common misconception about the flu shot.

Rosa Parks began advocating for equal rights for African Americans in the 1940s, when she was a young woman. She organized community meetings, helped find lawyers for those who had been falsely accused of crimes, and assisted citizens in navigating the cumbersome and discriminatory voter registration process. However, Parks did not become well known until 1955, when she refused a bus driver's order to relinquish her seat to a white passenger. She then helped lead a successful boycott against the bus company. In the following years, Parks continued her activism, eventually working for a U.S. congressman. In 1987, she cofounded the Rosa and Raymond Parks Institute for Self Development, an organization dedicated to training and inspiring young people. Having devoted her life to the quest for justice, Rosa Parks is considered an American hero.

It compares Parks's activities early in life with her later work.

Rosa Parks began advocating for equal rights for African Americans in the 1940s, when she was a young woman.

It establishes when Parks began her activism.

The creation of manatee protection zones—areas that boaters must avoid or drive through more slowly—has been particularly effective.

It highlights a successful conservation strategy.

These images last only a few minutes at most before dissipating, and they are typically not entirely accurate.

It highlights the limitations of eidetic memory.

Millions of years ago, volcanic eruptions released a large amount of mineral-rich water into the cave, now known as the Cave of Crystals.

It identifies a condition that led to the formation of gypsum crystals.

Many workers were trapped inside; the rickety fire escape collapsed, and the only accessible exit had been locked.

It identifies the conditions that led to the great loss of life in the fire.

Flu Confusion Though proven effective in flu prevention, the influenza vaccine, or "flu shot," won't keep you from contracting the stomach flu. This is because the name "stomach flu" refers to a condition that is completely different from the ordinary flu, or influenza. In common parlance, "stomach flu" typically refers to a condition called gastroenteritis, in which the digestive tract becomes irritated. There is no vaccine to prevent gastroenteritis, which may be caused by a variety of viruses, bacteria, other organisms, and certain foods. Common symptoms include stomach pain, vomiting, and diarrhea. The "regular" flu, on the other hand, results from a strain of the influenza virus. Common symptoms include fever, cough, congestion, fatigue, and muscle aches. Stomach upset may also occur, but this is not as common as the other symptoms.

The flu and the stomach flu are separate conditions, each with its own causes and symptoms.

Sweden's Pronoun Solution Many languages have what some consider a significant limitation: they lack a gender-neutral third-person singular pronoun. When using a pronoun to refer to a person, speakers must either specify a gender or say something like "he or she." Not only can this be cumbersome, but it means that there is no good pronoun choice when a person's gender is unknown to the speaker. Sweden addressed this problem in 2015 by adding a new pronoun—hen—which refers to a person without specifying a male or female gender. Hen does not replace the Swedish pronouns han, meaning "he," or hon, meaning "she"; it simply adds another option. Thus, Swedes now have their choice of three, rather than two, singular pronouns for referencing individuals.

To address the lack of a gender-neutral pronoun that refer to a person, Sweden created a new pronoun, hen, which does not specify gender.

A Bear in the Army While traveling through Iran during World War II, a group of Polish soldiers encountered a young boy with an intriguing possession available for trade: a bear cub. The soldiers promptly bought the cub, named him Wojtek, and took him to join the rest of their unit. The playful bear adapted well to military life. However, when the unit was sailing for Italy, officials refused to allow the bear on the ship, saying that only soldiers were permitted. The solution was both obvious and extraordinary: the soldiers arranged for Wojtek to be officially enlisted in the army. As an enlisted soldier, he could accompany the men on their mission. "He didn't receive money," recalled one former soldier in an interview with the BBC World Service, "but was officially a Polish soldier." Wojtek remained with the company throughout the war and even helped carry heavy crates of ammunition to soldiers on the battlefield.

A Polish army unit grew so fond of a pet bear that they enlisted the animal in the army in order to bring him aboard a military ship.

Rap Goes Mainstream One night in 1979, singer and record producer Sylvia Robinson encountered a new performance style while visiting a nightclub in Harlem, a neighborhood in New York City. The DJ was rapping—talking in rhyme as he spun his records. The technique had its roots in a performance style called toasting, brought to the city by Jamaican immigrants, in which DJs talked to the audience during instrumental portions of recorded songs. That night in Harlem, as her record label teetered on the brink of bankruptcy, Robinson was inspired to think outside the box; after all, she needed a hit. Could rap become a commercial success? Believing it was worth a try, Robinson assembled a group called the Sugarhill Gang and produced "Rapper's Delight," the first commercial rap single. As Robinson had hoped, it was a smashing success, launching rap into the mainstream.

An innovative performance and financial need led Sylvia Robinson to produce the first commercial rap single.

Maggots as Medicine While serving in World War I, surgeon William Baer noticed something curious: soldiers whose wounds were infested with maggots (the wormlike immature larval form of flies) recovered faster than those whose wounds were maggot-free. Although Baer was not the first military doctor to take note of this phenomenon, he was the first to apply his observations to the problem of nonhealing wounds. In the 1920s, he began treating hospital patients' serious wounds with maggots. The maggots consumed dead tissue, effectively cleaning the wounds and helping the patients heal. While maggot therapy fell out of favor after bacteria-killing antibiotic medications were introduced, it has made a comeback in modern medicine. Many clinicians have found that the treatment can be effective for certain types of severe wounds that do not respond to antibiotics.

As noted by William Baer, some severe wounds can be effectively treated with maggots, which consume dead tissue.

The Triangle Shirtwaist Fire The early twentieth century was a difficult time for New York City's garment workers, many of whom were immigrant women who worked long hours in dangerous factories for poverty-level wages. After the garment workers went on strike in 1910, some factory owners improved their workplaces and allowed employees to join unions. The Triangle Shirtwaist Factory was a notable exception: the owners resisted a union and continued to maintain shoddy working conditions. On March 25, 1911, a fire broke out at the factory. Many workers were trapped inside; the rickety fire escape collapsed, and the only accessible exit had been locked. In all, 146 workers perished. The tragedy sparked public outrage about labor injustices, and citizens pressed their government to take action. Consequently, over the next few years, New York's legislature passed many laws designed to improve workplace safety.

Because of the Triangle Shirtwaist Factory fire, the government began to regulate working conditions in factories.

Chimpanzees and Humans Humans and chimpanzees descended from a common ancestor, a gorilla-like creature that lived millions of years ago. Even in light of this fact, it is astonishing to learn just how similar humans and chimpanzees are when it comes to their genes. In fact, the two species' DNA differs by only 1.2 percent. A laboratory comparison of chimp and human chromosomes—strands of DNA each containing approximately 1,100 genes—reveals a strikingly similar pattern. Perhaps surprisingly, chimpanzees are genetically closer to humans than to gorillas. Admittedly, the apparently small 1.2 percent variation between chimps and humans still corresponds to forty million differences in DNA base pairs. Still, the tremendous genetic similarity indicates that the chimpanzee (along with the bonobo, another ape) is our closest living relative.

Chimpanzees and humans have surprisingly similar DNA.

Muggsy Bogues: Standing Tall Tyrone "Muggsy" Bogues played in the National Basketball Association (NBA) for fifteen seasons, most notably for the Charlotte Hornets. A popular player, he was known for his incredible agility and speed on the court. But when he was young, many people thought he would never be able to play professional basketball. The reason? At five feet three inches tall, Bogues was considered too short to master the game. Even after he won a basketball scholarship and began playing for Wake Forest University, commentators announcing the games mocked him. The turning point came when Wake Forest went up against North Carolina State University on national television. During that game, Bogues earned twenty points and had ten assists (passes to another player who then scores a basket), silencing his critics. When he was drafted by the Washington Bullets in 1987, he became the shortest NBA player in history—a personal and professional triumph.

Despite others' negative ideas about his small size, Muggsy Bogues had a successful career as a professional basketball player.

Discovering Penicillin In 1928, bacteriologist Alexander Fleming was studying Staphylococcus, a type of bacteria that can cause serious infections. He grew the bacteria in small containers called petri dishes. One day he noticed that mold had accidentally been introduced into one of the petri dishes and was startled to see that there were no bacteria around the moldy area. Fleming correctly deduced that the mold had had an antibacterial effect; somehow it had prevented the bacteria from growing. Fleming identified the mold as the strain Penicillium notatum and called its antibacterial ingredient—which he was unable to successfully extract—penicillin. Fleming's discovery of penicillin's unique effect led to the eventual finding that it could be used to treat infections. Several years after his initial observation, scientists built on his work by extracting penicillin from mold and demonstrating its effectiveness as a treatment.

Fleming's chance observation of the antibacterial properties of mold led to the discovery of penicillin as a treatment.

New Teen Literature S. E. Hinton's novel The Outsiders, published in 1967, differed radically from earlier teen novels. Most novels for teens projected a clean-cut, problem-free version of youth, with romantic love and sports as common topics. In contrast, The Outsiders dived into a much grittier reality, exploring complex and often disturbing issues, such as isolation, violence, and economic inequality. The Outsiders was itself authored by a teenager: Hinton was seventeen at publication. Her frustration with the existing literature was her impetus for writing the book. "There was only a handful of books having teenage protagonists: Mary Jane wants to go to the prom with the football hero and ends up with the boy next door and has a good time anyway," she told The New Yorker magazine in 2014. "I couldn't see anything going on in those books that had anything to do with real life."

In contrast to most previous novels written for teenagers, Hinton's The Outsiders deliberately dealt with difficult issues.

Bringing Books to People When Mary Lemist Titcomb became the first librarian of Maryland's Washington County Free Library in 1901, she was determined to ensure convenient, widespread access to library materials. To this end, she established dozens of small branch libraries at churches, post offices, and stores. While the plan was successful, Titcomb felt that some residents of isolated rural areas still found it hard to justify a trip to one of those still-distant locations, much less to the main library itself. Some people weren't even aware that the branches existed. Titcomb decided to address this situation by delivering books directly to peoples' homes. To achieve this goal, she designed the first horse-drawn book wagon. The wagon, with space for a variety of books, made its first trip in 1904. The project was so successful that it inspired other libraries to start their own mobile library programs.

In her quest to expand library access, Mary Titcomb designed the first mobile library.

That night in Harlem, as her record label teetered on the brink of bankruptcy, Robinson was inspired to think outside the box; after all, she needed a hit.

It explains why Robinson was open to taking a risk on rap.

After setting up camp in an abandoned bakery, they spent five months excavating a secret tunnel that wound its way under the wall to an East Berlin apartment building.

It identifies a method used to rescue loved ones.

However, when the unit was sailing for Italy, officials refused to allow the bear on the ship, saying that only soldiers were permitted.

It identifies the problem the soldiers faced in attempting to keep the bear.

Despite these concerns, the board of trustees chose a hearing president.

It identifies the source of conflict between the university and its students.

The islanders didn't attend special sign language classes; they simply learned the language as they grew up.

It illustrates how natural and widespread the use of sign language was on the island.

Unlike the Bolsheviks, they favored a gradual transition of power, and they preferred to avoid violent means of gaining control.

It illustrates one way that the Mensheviks differed from the Bolsheviks.

Because of the Magna Carta, the barons of England, along with civic and church leaders, began having regular meetings with the king to discuss matters such as proposed new taxes.

It illustrates the effect of the Magna Carta on English governance.

A Step for Democracy One of the most important years in the history of democracy was 1215, when King John of England signed a document called the Magna Carta. Among other things, the Magna Carta made it illegal to buy or bribe one's way out of legal trouble—previously a common practice. More important, the Magna Carta made it clear that the king of England could not change laws or impose taxes without the people's consent. Because of the Magna Carta, the barons of England, along with civic and church leaders, began having regular meetings with the king to discuss matters such as proposed new taxes. The barons were meant to represent the interests of the people who lived on their land. This practice—revolutionary at the time—laid the groundwork for the modern political system known as representative democracy. Representative democracy allows elected officials to speak for the interests of their constituents, the citizens they represent.

The Magna Carta set the stage for democratic governments by affirming the rights of citizens to participate in government.

The War of the Worlds Hoax "Martian cylinders are falling all over the country!" proclaimed a voice on the radio. It was Halloween night, 1938, and Orson Welles was in the midst of a performance of the H. G. Wells story The War of the Worlds. Most listeners were aware that it was fiction; after all, an explanatory announcement had been given at the program's beginning. However, listeners who tuned in late missed the program's introduction. In addition, Welles was a magnificent performer, interspersing his "report" with sound effects and eyewitness "interviews." Consequently, some audience members believed they were hearing a real news account. They listened in increasing terror as Welles described the "invading army from the planet Mars." Fearing for their lives, some took refuge in their basements or fled their homes. When Welles concluded the show with the admission that it had been a Halloween prank, reactions ranged from amusement to fury.

As a result of a news-style radio performance, some War of the Worlds listeners believed the broadcast to be true.

Rosa Parks: Lifelong Activist Rosa Parks began advocating for equal rights for African Americans in the 1940s, when she was a young woman. She organized community meetings, helped find lawyers for those who had been falsely accused of crimes, and assisted citizens in navigating the cumbersome and discriminatory voter registration process. However, Parks did not become well known until 1955, when she refused a bus driver's order to relinquish her seat to a white passenger. She then helped lead a successful boycott against the bus company. In the following years, Parks continued her activism, eventually working for a U.S. congressman. In 1987, she cofounded the Rosa and Raymond Parks Institute for Self Development, an organization dedicated to training and inspiring young people. Having devoted her life to the quest for justice, Rosa Parks is considered an American hero.

As an activist and a leader, Rosa Parks devoted her life to civil rights.

How Fevers Help Fevers can be uncomfortable, but the upside is that they are evidence of the body's attempt to heal. When a person develops an infection, a section of the brain called the hypothalamus recognizes a threat. The hypothalamus responds by increasing the body's temperature. Since both viruses and bacteria are temperature sensitive, this environment helps prevent them from multiplying. In addition, scientists have found evidence that fevers may increase the number of certain lymphocytes—white blood cells that are part of the immune system. Increased lymphocytes make the body more effective in destroying infected cells. Of course, this doesn't mean that high fevers don't warrant medical treatment. In fact, another reason that fevers are beneficial is that they serve as warning signs, indicating that something is amiss.

Fevers are useful because they help the body fight infections.

Different Kinds of Leaders Mongol leader Genghis Khan was one of the world's most formidable conquerors. Born around 1162, Genghis amassed an army of thousands of men and united the nomadic tribes of Mongolia under his rule. Next, he turned his attention to central Asia and China. Using brilliant military tactics—and extreme cruelty—he expanded the Mongol Empire to an area approximately the size of Africa. Kublai Khan, Genghis's grandson, conquered China and was a powerful leader in his own right. Yet unlike his grandfather, Kublai is remembered for governing his acquired territories fairly, rather than dominating them with military might. Instead of exploiting the people of China, Kublai sought to accommodate and embrace Chinese culture. He became the first Mongol to rule over all of China, and he founded the country's Yuan Dynasty.

Genghis Khan ruled with fear and might, while Kublai Khan governed with regard for the people he ruled.

Transforming Children's Television Mister Rogers' Neighborhood, created by Fred Rogers, first appeared on national television in 1968. Rogers developed the program in order to address what he perceived as a deficiency in television programming for young children; specifically, he viewed traditional programming as neglectful of social and emotional issues. One year after the show's national debut, another new public television children's program was introduced: Sesame Street, the brainchild of Joan Ganz Cooney and Lloyd Morrisett. Like Rogers, Coone and Morrisett were dissatisfied with the options available for young children. They created Sesame Street in order to add an educational option to children's programming. While Sesame Street was more academically focused than Mister Rogers' Neighborhood, both programs were unusual in their tackling of complicated issues, such as divorce, disability, and the human impact of natural disasters, which were typically ignored by traditional children's television shows.

Mister Rogers' Neighborhood and Sesame Street were television programs that both dealt with issues previously unaddressed in children's programming.

Titcomb decided to address this situation by delivering books directly to peoples' homes.

It describes Titcomb's plan to improve on her original solution.

In fact, the word synesthesia, derived from Greek, literally means "sensations together.

It describes the word's origin to reinforce the nature of the condition.

The same person's slow mode might decide (after a moment) that the spider is not actually a threat.

It illustrates the slow mode using a specific example.

Most novels for teens projected a clean-cut, problem-free version of youth, with romantic love and sports as common topics.

It sets up the contrast between earlier teen novels and The Outsiders.

Admittedly, the apparently small 1.2 percent variation between chimps and humans still corresponds to forty million differences in DNA base pairs.

It specifies the extent of the genetic similarity.

Synesthesia: Mixed-Up Senses We generally think of our senses as having completely separate functions. This is a logical assumption; after all, we can't see with our ears or taste with our eyes. For people with a condition called synesthesia, however, senses can be conflated, or mixed together. In fact, the word synesthesia, derived from Greek, literally means "sensations together." This combining of senses causes interesting sensory experiences. For example, some synesthetes (people who experience synesthesia) imagine tasting a particular flavor whenever they hear a certain sound. Others "hear" a sound in their minds whenever they touch a certain object. The most common types of synesthesia involve colors. In one common subtype, for instance, synesthetes associate different colors with different letters of the alphabet. Typically, synesthetes are not bothered by their synesthesia; in fact, many enjoy it.

People with synesthesia experience a fusing of different senses.

Crystal Caves of Mexico While excavating a tunnel under Mexico's Naica Mountain, silver miners were stunned to discover a cavern full of luminous, elongated underground crystals. Composed of the mineral gypsum, some of the crystals extended as far as thirty-six feet into the air. What accounted for these amazing mineral structures? Millions of years ago, volcanic eruptions released a large amount of mineral-rich water into the cave, now known as the Cave of Crystals. One of those minerals was anhydrite, which slowly changes into gypsum when exposed to water. Originally, the cave was too hot for this change to take place. Eventually, however, the temperature cooled to about 136°F, and gypsum crystals began to form. This process continued as the temperature and other conditions in the cave remained constant for thousands of years.

Specific conditions in the Cave of Crystals led to the development of the enormous crystal structures found there.

Fast or Slow? The human brain can be thought of as having two different modes. Daniel Kahneman, an influential psychologist, labels one the "fast" mode of thinking and the other the "slow" mode. In the fast mode, according to Kahneman, we make decisions using our intuitions and emotions. The sight of a spider may activate the fast mode and lead a person to scream. The same person's slow mode might decide (after a moment) that the spider is not actually a threat. In the slow mode, we make decisions using reason and logic. The slow mode is helpful in complex situations where the right choice is not always clear. However, because the slow mode uses more mental resources than the fast mode, our brains can't rely on it all the time. The instincts that drive the fast mode of thinking save us a great deal of time and energy.

The "fast" mode of thinking is helpful for reacting quickly, while the "slow" mode is helpful for analyzing complex situations.

The Race to Space The first artificial Earth satellite, Sputnik 1, was launched into space by the former Soviet Union on October 4, 1957. As momentous as this event was for science, it was a blow to the United States. Americans feared that the launch indicated a technological gap between the United States and the Soviet Union, America's rival in the Cold War at the time. With a technological edge, they worried, the Soviets could dominate the exploration of space or develop superior weapons. This widespread concern spurred the U.S. government to combine several separate organizations into a new, better-funded organization called the National Aeronautics and Space Administration, or NASA. NASA officially began operations on October 1, 1958. Four months later, the U.S. successfully launched its own satellite, Explorer 1.

The launch of Sputnik 1 by the Soviet Union prompted the United States to create NASA.

Yet unlike his grandfather, Kublai is remembered for governing his acquired territories fairly, rather than dominating them with military might.

It draws a contrast between the two leaders' styles of governance.

Even after he won a basketball scholarship and began playing for Wake Forest University, commentators announcing the games mocked him.

It emphasizes the extent to which people focused on his small size.

Not only can this be cumbersome, but it means that there is no good pronoun choice when a person's gender is unknown to the speaker.

It identifies a limitation of gender-specific pronouns.

Sign Language on Martha's Vineyard While researching Martha's Vineyard, an island off the coast of Massachusetts, anthropologist Nora Groce made a remarkable discovery. She learned that from the 1600s until the early 1900s, almost all the residents of two towns on the island—West Tisbury and Chilmark—were fluent in sign language. During these years, the towns had an unusually high population of deaf people, due to a form of hereditary deafness that ran in many Martha's Vineyard families. Consequently, a bilingual society developed. The islanders didn't attend special sign language classes; they simply learned the language as they grew up. One of the most interesting aspects of the phenomenon was that, in the absence of language barriers, deafness was not perceived as a disability. As one Vineyarder explained to Groce, deaf people were seen as "just like everyone else."

The high incidence of deafness on Martha's Vineyard led to the development of a bilingual society.