Why do onions make you cry?

Answer 1

The chemical that gives onions a lot of their flavor is also an eye irritant- which means they will sting your eyes, and your eyes make the tears trying to get the chemical out.

The only way I've found to keep my eyes from tearing up is to rinse the onion under cold running water so this chemical doesn't have a chance to get in the air- and into my eyes. This after I quarter the onion, and sometimes after other cuts.

Answer 2

it's some sort of chemical or something that irritates the eye.
it's not a boo-hoo sort of cry, it's just an irritant that causes your eyes to water.