How do you cut an onion without crying?

Cutting an onion ruptures cells, mixing enzymes with sulfur compounds and producing syn-propanethial-S-oxide, a volatile gas that reaches your eyes and reacts with the moisture there to form a mild sulfuric acid, triggering the tearing reflex. Every effective remedy works by either slowing that reaction or moving the gas away from your face, which is why so many folk methods fail: they do neither.

The tactics with real mechanisms behind them. Refrigerate or briefly freeze the onion before cutting, since the reaction proceeds more slowly at lower temperature and the compound is less volatile. Use a very sharp knife, which slices cells cleanly rather than crushing them and releases less enzyme. Cut near a running exhaust fan, an open window, or a small fan pointed across the board, which carries the gas away before it reaches you. Leave the root end intact until last, since sulfur compounds concentrate there. And swimming or safety goggles work completely if you are willing to wear them, which is the actual barrier.

The methods that do not hold up under scrutiny include holding bread in your mouth, cutting under running water, which is also a knife safety problem, chewing gum, and lighting a candle nearby. Onion variety matters more than most tricks: sweet onions such as Vidalia contain fewer sulfur compounds and are noticeably milder to cut, while yellow and especially strong storage onions are the worst offenders. Contact lens wearers often report less irritation because the lens provides a partial barrier. If none of this is practical, buying pre-chopped onion is a legitimate answer, at the cost of some flavor since the volatile compounds that make you cry are related to the ones that make onion taste like onion.