We can form 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
This conditional is used when the result will always happen.
If water reaches 100 degrees, it always boils.
It's a fact.
The result of the 'if clause' is always the main clause.
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)
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)
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.