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Q: Why do onions make you cry?

Cutting onions releases a compound called lachrymatory factor, which is really just a fancy way of saying tear-causing compound. The exact process of how that compound was made was a bit of a mystery until recently. Chemists from Case Western University have now described the complex chemical reaction that occurs to create the stinging compound.

Cutting an onion releases a sulfenic acid, whose chemical bonds are repositioned by an enzyme to form lachrymatory factor. The factor is released into the air as a gas, irritating the nerves in the eyes, causing the production of tears.

Chilling the onion before cutting it can help minimize the effect. Other people wear goggles to protect their eyes or cut the onion in a bowl of water. If all that is too much effort, you could track down a tearless onion, invented in Japan, that has been irradiated to deactivate the enzyme that starts the process.

— Markian Hawryluk, The Bulletin