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. 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 '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, not at sea level)
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)