Trilobites: Why Onions Make You Cry

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When we cut onions, two substances combine in a chemical reaction, releasing a gas that causes us to cry, similar to that of tear gas.

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Karsten Moran for The New York Times

Trying to figure out why humans cry is exhausting. We cry about death, violence, breakups, abandoned puppies, sweet kisses and words charged with all kinds of meanings. We don’t cry when we should, and we cry for no reason. But let’s take a moment to appreciate the clarity of crying while cutting onions.

Onions make us teary because a reaction in the onion releases a chemical called lachrymatory factor, or LF, that irritates our eyes. Simply peeling an onion won’t make your eyes water. But if you chop, cut, crush or smash one — boohoo. The onion’s cells break open, allowing two normally separated substances to combine. Linked together like pieces of a puzzle, they become a potent chemical weapon.

“It turns into a gas. It hits your eyes, and then it hits your sensory nerves in your eyes and causes them to tear up,” said Josie Silvaroli, an undergraduate at Case Western Reserve University in Ohio who helped describe how these pieces fit together, structurally speaking, in a paper published in July in the journal ACS Chemical Biology. “It’s similar to tear gas.”

Lachrymatory factor evolved as a defense mechanism, protecting onions against microbes and animals like us, even if we’ve learned to bear tears for the sake of flavor. Damaging an onion basically causes it to ramp up its defenses: as cells break, the chemical reaction is unlocked.

Inside the intact cells of an onion, a molecule called sulfenic acid precursor floats around the watery filler like a napping human in a lazy river. Also floating in that cytoplasm are little sacs called vacuoles, containing a protein called alliinase, which is like a little drill sergeant of the process.

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