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Why do eyes tear when we work with onions?

When an onion is cut, its pierced cells release a sulfur compound, thio-propanal-s-oxide, through a series of rapid chemical reactions. This substance is irritating to the eyes.

Sources: McGrayne, Sharon Bertsch. 365 Surprising Scientific Facts, Breakthroughs, and Discoveries, p. 132; Pittsburgh Post Gazette, January 22,1995, p. JI.

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