Why do we cry when we cut onions?
Onions produce the chemical irritant known as syn-propanethial-S-oxide. It stimulates the eyes' lachrymal glands so they release tears.
Cutting an onion opens up its cells, allowing enzymes normally kept separate to mix. This chemical reaction releases one of two gases, syn-propanethial-S-oxide or lachrymatory factor, which irritate the eyes. Your body responds to this irritation by increasing production of tears.
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