Why do onions make you cry?

Cutting an onion starts a short chemical chain. Damaged onion cells let the enzyme alliinase meet sulfur-containing compounds. The reaction produces sulfenic acids, and lachrymatory factor synthase (LFS) turns them into propanethial-S-oxide, a volatile irritant. 12

The irritant reaches moisture on the eye, where acid forms and nerves trigger tears. Tears dilute and wash away the irritant. 34

The useful memory is simple: cut, mix, gas, tears. The tear trigger appears after the cut because the knife brings the onion’s reaction partners together. 4