Why do onions make you cry?
The Real Answer
Cutting an onion breaks its cells and starts reactions that produce a volatile irritant called the onion lachrymatory factor, syn-propanethial-S-oxide. When that vapor reaches your eyes, it irritates them and triggers reflex tearing to wash the irritant away. The tears are not evidence that the onion is creating a meaningful sulfuric-acid chemical burn on your eyes.
An onion's final act before being chopped into dinner is chemical warfare. You cut it open and it releases a gas that goes straight for your eyeballs. The onion knows it's going down, but it's going to make you look like you're the one who's upset about it.
The Dunce Dynamics Institute Examines Why People Ask This
Onion tears happen so reliably that it is natural to assume the eye is being chemically burned. The actual irritant is a volatile tear-producing compound released after the onion's cells are damaged.