Why do onions make you cry?
Onions defend themselves with chemistry. Intact, they store sulfur compounds and an enzyme separately; cutting ruptures the cells and mixes them, triggering a reaction that produces a volatile gas called syn-propanethial-S-oxide. That gas wafts up, reaches the moisture on your eyes, and forms a small amount of sulfuric acid, which stings, and your eyes respond by producing tears to flush the irritant, hence the crying. It is a genuine mild chemical irritation, not emotion or imagination.
The kitchen fixes all target the reaction. Chilling the onion for 15 minutes first slows the enzyme and reduces the gas released. A truly sharp knife ruptures fewer cells than a dull crushing one, so it produces less irritant. Cutting near a running vent or fan carries the gas away, and cutting the root end last helps because the root concentrates the sulfur compounds. Goggles work if you are willing to look ridiculous, which serious cooks occasionally are. Scientists have even bred low-tear onions by dialing down the responsible enzyme.
Cutting an onion releases an enzyme that builds a sulfur gas; it reaches your eyes, reacts with tears to form mild sulfuric acid, and your eyes flush it out. Chill the onion and use a sharp knife to cut the reaction.
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https://frequentlyaskedquestions.us/q/why-do-onions-make-you-cry/Everybody Asks. (2026, July 28). Why do onions make you cry? https://frequentlyaskedquestions.us/q/why-do-onions-make-you-cry/“Why do onions make you cry?” Everybody Asks, 28 July 2026, https://frequentlyaskedquestions.us/q/why-do-onions-make-you-cry/.“Why do onions make you cry?” Everybody Asks. Last modified July 28, 2026. https://frequentlyaskedquestions.us/q/why-do-onions-make-you-cry/.This page summarizes the primary sources listed above. For academic or encyclopedic work, cite those primary sources directly wherever possible.