Whether sautéed, grilled, caramelized or raw, onions are a staple in many U.S. households; the average American consumes 20 pounds (9 kilograms) of onions per year. But their coveted flavor comes at a price: Whoever chops them may soon feel tears running from their burning eyes. But why does slicing onions make you cry?

The culprit is called the lachrymatory factor, a chemical that irritates the eyes. When the onion is intact, a group of compounds called cysteine sulfoxides are kept separate from an enzyme called alliinase. But when you slice, dice or crush the onion, the barrier separating the compounds and enzyme is broken. The two come together, setting off a reaction: The alliinase causes the cysteine sulfoxides to become sulfenic acid. 


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