Why Do Onions Make You Cry? The Sneaky Chemistry Behind Kitchen Tears

Your onion is basically booby-trapped

An onion just sitting on the counter won't make you cry. Inside its cells, it keeps certain sulfur compounds and the enzymes that react with them in separate compartments. Everything stays calm until a knife shows up.

When you slice an onion, you break open those cells and the ingredients mix. An enzyme called alliinase turns the sulfur compounds into new chemicals, and a second enzyme quickly turns one of those into the tear-making gas. Scientists think this is the onion's way of defending itself from hungry animals. It's a tiny, very effective prank.

Then the gas goes straight for your eyes

The gas, syn-propanethial-S-oxide, drifts up from the cutting board. When it reaches your eyes, it irritates nerves on the surface, and your body responds the way it does to anything that bugs your eyes: it makes tears to flush it out.

So those aren't sad tears. They're your eyes doing their job. Your face just happens to look like you're watching the end of a very emotional movie while making spaghetti sauce.

The discovery won a very silly prize

For a long time, scientists thought alliinase alone was behind onion tears. In 2002, researchers in Japan reported in the journal Nature that a separate enzyme, which they named lachrymatory factor synthase, is what actually makes the tear-causing gas.

That work later earned the team an Ig Nobel Prize in Chemistry in 2013. The Ig Nobels honor research that "makes people laugh, then think," and onion tears definitely qualify.

Garlic, by the way, is a close cousin of the onion, but its main sulfur compound is a little different. When you cut garlic, it turns into allicin, the stuff behind garlic's famous smell, instead of tear gas. That's why mincing garlic makes your kitchen smell amazing without turning you into a puddle.

How to cry a little less

Use a sharp knife. A clean cut breaks fewer cells than a dull blade that crushes the onion, so less gas gets released. Slow down, too: a 2025 study found that faster chopping sprays more onion juice into the air. (Some cooks swear by chilling onions first, but that same study found it didn't cut down the spray.)

Good airflow helps too, like cutting near an open window or a running vent fan. And if you really want to win, kitchen goggles work, because the gas can't reach eyes it can't touch. As for old tricks like holding bread in your mouth or chewing gum, there isn't much evidence they do anything besides make you look funny.