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

See this page about the first conditional to learn about the difference between the first and the zero conditionals. The first conditional is about a specific situation, but the zero is talking in general.