Why do onions make us cry when we cut them?
Short answer
Cutting an onion breaks its cells and starts a chemical reaction. The reaction makes an irritant that spreads through the air. When it reaches the eye, a protective reflex causes more tears.
How it works
Onion cells contain sulfur-containing compounds and enzymes. An enzyme helps a chemical reaction happen. Cutting the onion breaks the cells, so their contents mix and a series of reactions begins.
The reactions produce a volatile irritant, meaning it can spread through the air. It reaches the surface of the eye and stimulates nerves there. Those nerves signal the tear glands to release more tears, which help wash the irritant away.
Are these emotional tears?
No. Onion tears are reflex tears: they start automatically when the eye is irritated. Emotional tears have a different trigger. For more about how tears protect the eye, see Why do tears form?
Key point
The tear-triggering compound forms after cutting damages the onion’s cells. One main compound is called the onion lachrymatory factor; its chemical name is (Z)-propanethial S-oxide. You do not need to memorize the long name: it forms after cutting, travels through the air, and irritates the eye.