Why do onions make us cry when we cut them?

Onions absorb sulfur from the soil during growth. They store it in the form of sulfur-containing amino acids called sulfoxides. These are harmless—until the onion is disturbed. This sulfur is key to the tear-jerking reaction.

When you slice an onion, you're not just cutting vegetables — you're triggering a chemical defense system. Slicing damages onion cells, releasing enzymes like alliinase that mix with the stored sulfur compounds.

The alliinase enzyme reacts with sulfoxides to form sulfenic acids, an unstable group of molecules. This reaction happens almost instantly. a critical step in producing the compound that causes tears.

Sulfenic acid molecules quickly rearrange to become Syn-Propanethial-S-oxide, a volatile sulfur compound. It's airborne, meaning it can easily float up and reach your eyes, acting like a natural irritant.

This tear-inducing gas rises off the onion and into the air. As it floats upward, it reaches your eyes and dissolves into your tear film . the thin moisture layer covering your eye surface. That’s where the irritation begins.

Once in your tear film, the gas forms tiny amounts of sulfuric acid through a mild chemical reaction. This stings your eyes and activates pain receptors, triggering a powerful reflex: your tear glands go into overdrive.

Your body tries to flush out the irritant by producing tears. The more gas your eyes are exposed to, the more you cry. This reflex is your body's natural way of protecting your eyes from foreign chemicals.