Why cutting onions makes you cry

Chopping an onion breaks open its cells. Enzymes that were kept apart suddenly meet sulfur-rich compounds, and the mix creates a volatile gas called syn-propanethial-S-oxide.1

That gas drifts up to your eyes. On the moist surface of the eye it forms a mild acid, so your brain triggers tears to rinse the irritant away.2 The cascade is a two-step enzyme path: alliinase starts the sulfur chemistry, then lachrymatory-factor synthase builds the tear gas itself.3

Onions use the gas as a defense against animals — botanical warfare that only starts once something cuts into the bulb.4 Cooling the onion before you chop slows the enzymes and softens the sting.

Sources: Britannica · Chemistry World · Nature · ACS Molecule of the Week

Image material: A person cutting an onion