The Zero Conditional

The 'if' in this conditional can usually be replaced by 'when' without changing the meaning

For example: If water reaches 100 degrees, it boils. (It is always true, there can't be a different result sometimes).

We can make a zero conditional sentence with two present simple verbs (one in the 'if clause' and one in the 'main clause'):

If + present simple, .... present simple.

Here are some more examples:

If people eat too much, they get fat. If you touch a fire, you get burned. People die if they don't eat. You get water if you mix hydrogen and oxygen. Snakes bite if they are scared If babies are hungry, they cry

This conditional is used when the result will always happen

So, if water reaches 100 degrees, it always boils. It's a fact. I'm talking in general, not about one particular situation. The result of the 'if clause' is always the main clause.

the zero is talking in general.

The first conditional is about a specific situation,

If I eat peanuts, I am sick.

(This is true only for me, maybe, not for everyone, but it's still true that I'm sick every time I eat peanuts)