Why do onions make you cry?
Cutting the onion mixes two chemicals it kept apart, and the reaction makes a volatile gas that turns mildly acidic on the wet surface of your eye.
Simple intuition
The plain reason, in everyday words
An onion sitting on the counter does nothing to your eyes. Pick it up, sniff it, hold it against your face and you feel nothing. That is because the two ingredients responsible are stored in separate compartments inside the onion's cells — a chemical in one place and the tool that transforms it in another. Cutting breaks the walls between them, and within seconds they mix and produce something neither was on its own: a small, light molecule that evaporates readily and drifts up toward your face. When it lands on the wet surface of your eye it reacts and produces a mild irritant. Your eye responds the way it responds to anything irritating, by flooding itself with tears to wash it away. The onion is not attacking you specifically. It is defending itself against being eaten, and you happen to be the thing eating it.
The tear-causing chemical is inside the onion waiting to get out.
It does not exist until the onion is cut. Two components stored separately react on contact, and the irritant is the product of that reaction.
Holding bread in your mouth or breathing through your nose helps.
The compound travels through the air and reacts on the surface of the eye. Anything that does not either reduce production or keep the gas away from your face has no mechanism by which it could work.
Garlic should do the same thing since it is closely related.
Garlic has the first enzyme but not the second one that makes the tear compound, so its version of the pathway ends in flavour and odour molecules instead.
The onion is chemically attacking you.
It is a damage-triggered defence aimed at anything that bites the bulb. That the product happens to be extremely effective against mammalian eyes is why we notice it, but nothing about the mechanism is specific to people.
It is a chemical reaction you can start and stop at will in your own kitchen, with an effect you cannot ignore. It also illustrates a design pattern that appears throughout biology and engineering alike: keep the dangerous thing in two harmless halves and only combine them at the moment of use. Two-part adhesives, binary chemistry and several drug delivery systems use the same idea for the same reason.
Who worked it out
The chemistry of Allium plants was opened up in 1944 when Chester Cavallito isolated allicin from garlic, showing that the pungent compounds form only on tissue damage rather than existing in the intact bulb.
What problem forced it
The onion's tear factor was identified as syn-propanethial-S-oxide in the 1970s, but it was assumed to arise spontaneously from an unstable intermediate.
How it changed since
In 2002 a Japanese research group identified lachrymatory factor synthase, showing a dedicated enzyme was responsible. Silencing its gene later produced tearless onions and confirmed the pathway by removing exactly the expected product.
How mustard and wasabi produce their heat
A different pair of chemicals with the same two-compartment architecture, acting on the same nerve receptor.
Why cooking onions makes them sweet
Heat destroys the enzyme before it can act, and separately breaks down sugars, which is why a cooked onion is a completely different ingredient.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.