Why do onions make you cry?
Cutting onions causes lachrymatory-factor synthase to be released, which combines with sulfur in the onion to form syn-propanethial-S-oxide. This is a volatile compound which easily turns into a gas and stimulates your lachrymal (tear) glands, causing you to cry.
Onions produce the chemical irritant known as syn-propanethial-S-oxide. It stimulates the eyes' lachrymal glands so they release tears.
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