Why cutting an onion makes you cry

The sting isn't in the onion until the knife goes in. A chemical defence, a gas that drifts up, and a few tricks that actually work.

A whole onion doesn't make anyone cry. You can hold it up to your eyes for as long as you like. The trouble starts with the knife, because the onion keeps the ingredients of its defence apart, in different parts of each cell, and only cutting brings them together.

When a cell breaks, an enzyme called alliinase meets sulphur compounds that were stored safely away and turns them into unstable sulphenic acids. A second enzyme, lachrymatory-factor synthase, turns those into a gas called syn-propanethial S-oxide. It is light, it rises from the cutting board, and it reaches your eyes within seconds.

On the surface of the eye the gas excites the nerve endings that sense irritation. The brain answers the way it answers dust or smoke: the tear glands open to wash the irritant away. The tears are a cleaning response, not a reaction to taste or smell.

Most of the old advice works because it slows the gas down or keeps it away from your face. A chilled onion releases less of it, since cold slows both the enzymes and evaporation. A sharp knife cuts cleanly instead of crushing, so fewer cells break. Cutting near a vent or an open window carries the gas away.

The same chemistry is also why onions are worth the trouble. Softened by heat, those sulphur compounds give cooked onions their sweetness and depth.