Few kitchen tasks are as reliably emotional as slicing an onion. You start with dry eyes and a cutting board, and a minute later you are blinking and reaching for a towel. The onion is not being dramatic for no reason, though. It is running a small chemical defense routine, and you happen to be standing right in front of it.

A defense that starts with a cut

An intact onion is fairly calm. Its cells store sulfur-containing compounds in one place and an enzyme in another. When a knife breaks those cells open, the two meet. The enzyme converts the stored compounds into reactive molecules, and a second enzyme turns some of those into a volatile gas known as syn-propanethial-S-oxide.

Plants did not develop this system to annoy cooks. For an onion growing in the ground, a broken cell usually means something is chewing on it, and an irritating gas is a useful way to discourage the chewer.

Why the gas reaches your eyes

Because the compound is volatile, it drifts up from the cutting board as you work. When it reaches the surface of your eyes, it stimulates sensory nerves there. Your body responds the way it responds to many irritants: it produces tears to rinse the surface clean. The stinging and the tears are a reflex, not a sign that anything is wrong with the onion or with you.

The more cells you break, the more gas is released. That is why fine dicing usually feels worse than cutting an onion into a few large wedges.

Small tricks cooks use

None of these methods is perfect, but many home cooks find that a few of them make a noticeable difference:

  • Use a sharp knife. A sharp blade slices cleanly through cells instead of crushing them, which tends to release less of the irritating gas.
  • Chill the onion first. A short rest in the refrigerator slows the chemical reactions and makes the gas less eager to rise.
  • Give the air somewhere to go. Working near an open window or under a running kitchen fan moves the gas away from your face.
  • Leave the root end for last. Many cooks believe the root area releases the most irritation, so they cut around it and discard it.
  • Work quickly and cover what you cut. The less time cut pieces sit on the board, the less gas reaches you.

Why cooked onions behave differently

Once onions are heated, the enzymes responsible for the gas stop working. That is part of why a pan of softened onions smells sweet and mellow rather than sharp. The same vegetable that made you cry on the cutting board becomes one of the gentlest flavors in the kitchen after a few minutes of slow cooking.

So the next time your eyes start to water, you can take some comfort in knowing exactly what is happening: a tiny plant defense, doing precisely what it evolved to do.