Why do onions make you cry?
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tl;dr
Cutting ruptures the onion's cells, mixing enzymes with sulfur compounds to make a volatile gas that turns into mild acid on your eyes — tears are the rinse cycle.
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Cutting an onion breaks its cells, and two chemicals that were kept apart suddenly mix. The result is a gas that drifts up, dissolves in the film of water on your eyes, and turns into a trace of acid. Your eyes flush it out the only way they can: tears.
The two-chamber trick
An intact onion keeps enzymes and sulfur compounds in separate compartments. The knife is what introduces them. Enzymes convert the sulfur compounds into an unstable acid, and a second enzyme flips that into a volatile gas — syn-propanethial S-oxide, the tear-maker.
From gas to tears
The gas rises, meets the water on your cornea, and forms a very dilute sulfuric acid. Nerves on the eye's surface report the sting, and your brain orders the tear glands to dilute the intruder. Crying is the correct response — it works.
The cold shortcut
Enzymes slow down in the cold. Ten to fifteen minutes in the fridge before chopping cuts the gas production sharply, and a sharp knife crushes fewer cells than a dull one. Chill it, then slice.
cited
Sources
- [01]Why do onions make you cry? — Poison Control
“Once syn-propanethial S-oxide reaches the eye, it dissolves in the tear film, creating a dilute solution of sulfuric acid on the eye's surface.”
- [02]syn-Propanethial S-oxide — American Chemical Society
“The lachrymatory factor is produced when onion cells are ruptured, releasing enzymes that convert sulfoxides into the volatile compound.”
people also ask
Keep asking
Does chilling an onion really help?
Yes — 10-15 minutes in the fridge slows the enzymes that make the tear gas, so far less of it reaches your eyes. A sharp knife helps too: fewer crushed cells, less gas.
Why don't cooked onions make you cry?
Heat destroys the enzymes before they can make the gas. That's also why fried onions turn sweet — the sharp sulfur chemistry is gone.
Do goggles or wet blades actually work?
Goggles block the gas outright. Wetting the blade or cutting near running water helps a little because the gas dissolves in water before it can climb to your face.
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