Why do onions make you cry when you cut them?

KohakuWhen you chop an onion, tears fall even though you aren't sad at all. That is the onion's doing.

The answer in 30 seconds

The cause of the tears is born the very moment you cut.

When you cut an onion, its cells break open. Compounds and enzymes that were stored separately mix together and produce a substance that irritates the eyes (a tear-causing compound). This substance drifts through the air, and when it touches the moisture on the surface of your eyes, it stings, so your eyes make tears to wash it away. The special enzyme that makes this substance was discovered in 2002 by a research group in Japan.

A red onion cut in half lengthwise, showing its cross-section — A red onion cut in half. Cutting breaks its cells and creates the substance that causes tears.Photo: Amada44 / Public domain / Wikimedia Commons

Kept apart inside the cells

According to a horticulture professor at the University of Wisconsin–Madison, onion cells keep sulfur-containing compounds and an enzyme that acts on them, called alliinase, in separate places. When the cells are broken, by a knife or by an insect taking a bite, the two mix and a reaction begins.

A special enzyme that makes the tear trigger

People used to think the tear-causing substance formed on its own partway through this reaction. But in 2002, a research group in Japan reported in the science journal Nature that they had found a special enzyme that makes it, called lachrymatory-factor synthase ("lachrymatory" means tear-causing). A guide from the U.S. Library of Congress also names this enzyme as the cause of the tears.

KohakuSo it was a research group in Japan that found the enzyme behind the tears. Isn't that interesting?

Eyes try to wash it away with tears

The substance that forms drifts through the air and stings when it touches the moisture on the surface of the eyes. According to an explanation from McGill University, the irritation sends a signal to the tear glands, and the eyes try to wash the irritant away with tears. Tears are a reaction that protects the eyes.

Figure: From cutting an onion to tears

Originally a way to protect itself

This reaction is thought to be the onion's way of defending itself against insects and microbes. To the onion, we are just like any other creature coming to take a bite when we cut into it with a knife. Cooked onions don't make you cry because heat stops the enzymes from working.

Want to know more?

Chilling an onion before cutting it is said to slow down the enzymes, so you are less likely to cry.
Sweet onion varieties grown in low-sulfur soil have a weaker reaction and are less pungent.
Onions get sweeter when you sauté them because heat creates sweet flavors and aromas and breaks down some of the sharp-tasting compounds.

KohakuCompounds released from broken cells irritate the eyes. They say chilling the onion before cutting it helps you cry less.

Quick quiz: test yourself

Why do onions make you cry when cut?

Show the answer

An eye-irritating compound forms
Onion juice splashes into your eyes
The sharp taste travels to your eyes

An eye-irritating compound forms Cutting breaks the cells, so compounds and enzymes mix and make a substance that irritates the eyes. Your eyes make tears to wash it away.

Sources

Curiosities: Why do onions make us cry when we cut them?University of Wisconsin–Madison News
An onion enzyme that makes the eyes water (Nature 419, 2002)Nature (Imai et al.)
Why does chopping an onion make you cry?Library of Congress, Everyday Mysteries
Why do onions make you cry when you cut them? And why are they sweet when you fry them?McGill University Office for Science and Society

This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.