Why do onions make you cry?

A knife turns an onion from quiet vegetable into a tiny chemistry lab. The onion is not storing a finished tear-triggering gas; it makes the irritant when its cells are damaged.

You cut, your eyes sting, tears arrive. The culprit is a volatile onion irritant called syn-Propanethial-S-oxide, which stimulates the tear glands. 1

Cutting breaks onion cells, letting alliinase and sulfur-containing precursors mix. The reaction makes sulfenic acids, including the intermediate that feeds the tear-making pathway. 2

The onion does not contain much ready-made sting. Lachrymatory-factor synthase, usually shortened to LFS, helps convert sulfenic acid into the volatile lachrymatory factor after tissue is damaged. 3

The sting is made on demand. Once that airborne molecule reaches the eye, it meets moisture and triggers a protective tear response to wash the irritant away. 4

What everyday "why" should the next card answer?