Why do onions make us cry?
Cutting an onion releases enzymes that build a tiny irritant gas. It drifts up, hits the cornea, and your eyes flush it out with tears.
knife breaks cells
enzymes mix
syn-propanethial-S-oxide forms
gas drifts up
eyes water
An onion is a chemistry kit waiting for a knife
Whole onions are calm. Their cells keep two ingredients apart: sulfur compounds in one place, an enzyme called alliinase in another.
When the knife smashes those compartments together, the enzymes get to work and a brand-new molecule, syn-propanethial-S-oxide, is built in seconds.
It’s the same trick a tear-gas grenade plays
intact onion (no smell)
cut onion (irritant cloud)
cut: lachrymatory
uncut: stable
The molecule is volatile, drifts upward, and reacts with the moist surface of your eyes to make a mild sulfuric acid. Your tear ducts respond by flushing it.
Cheats that work
Chill the onion (slows the enzymes). Use a sharp knife (fewer ruptured cells). Cut near a lit candle or running hood (the gas gets diverted). Or, frankly, wear swim goggles.