Why does cutting onions make you cry?

In an intact onion the chemical responsible does not yet exist. Its cells keep sulphur compounds and the enzymes that act on them in separate compartments. A knife breaks those walls, the two mix, and the reaction produces a volatile gas.

The gas drifts up, reaches your eyes and reacts with the moisture there to form a weak sulphuric acid. Your eyes register the irritation and flood themselves with tears to wash it away — which looks like crying.

1. Chill the onion before cutting — the gas is less volatile at lower temperature.
2. Use a sharp knife. A blunt one crushes more cells and releases more gas.
3. Cut near running water or moving air so the gas disperses before it reaches your eyes.
4. Breathe through your mouth and keep your face a little further back.

One popular tip does not work: cutting with your eyes shut. The gas still reaches them through closed lids, and shutting your eyes around a knife is a bad idea in itself. Glasses or contact lenses do help a little, though, by shielding the surface of the eye.