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Biology

What causes that strong smell when you cut an onion?

Quick fact

Onions release a gas called syn-propanethial-S-oxide (aka 'lachrymatory factor') that dissolves in your tears to form sulfuric acid, triggering a burning sensation that makes you cry.

Why this is interesting

An onion's tear-inducing compound is a volatile chemical weapon designed to deter pests—but it's not produced until you break the cell walls, creating a new compound that attacks your eyes on contact.