why do onions make you cry

Onions make you cry because cutting them releases a volatile gas that irritates your eyes, triggering a reflex tear response to wash it away.

Chemical reaction process. When an onion is sliced, cells break open and release an enzyme called alliinase, which reacts with sulfur compounds to form sulfenic acid. A second enzyme, lachrymatory factor synthase (LFS), then rearranges this acid into the volatile lachrymatory factor (syn-propanethial-S-oxide).
Eye irritation and reflex tearing. The lachrymatory factor is a volatile gas that easily vaporizes and travels through the air to your eyes. Upon contact with the cornea, it triggers sensory nerves and causes the lachrymal glands to produce tears as a protective, diluting mechanism.
Evolutionary defense mechanism. Scientists believe this chemical defense system evolved to protect the onion plant from being eaten by insects, animals, or parasites that might damage it.
Tips to reduce tearing. Common suggestions to reduce tearing include using a very sharp knife to minimize cell damage, chilling the onion before cutting to slow the chemical reaction, or cutting in a well-ventilated area.