What vegetable makes your eyes water when you cut it?

The familiar stinging and tearing sensation experienced when slicing into an onion is a fascinating result of a natural chemical defense mechanism. When the layers of an onion are cut, its cells are broken, releasing previously separated enzymes and sulfur-rich amino acids. These compounds quickly react with each other and with the air to produce a volatile gas.

Specifically, enzymes like alliinases break down amino acid sulfoxides, generating unstable sulfenic acids. These sulfenic acids then rapidly rearrange, thanks to an enzyme called lachrymatory factor synthase, into a chemical known as syn-propanethial S-oxide. This airborne compound is the primary irritant, or lachrymatory factor, that floats up from the cutting board and makes contact with your eyes.

Once syn-propanethial S-oxide reaches the moisture on the surface of your eye, it reacts to form a mild sulfuric acid. This acid irritates the sensitive nerves around your lacrimal glands, which are responsible for tear production. Your brain, perceiving this irritation as a threat, quickly triggers a reflex to produce tears as a protective measure, flushing out the offending compound and diluting the acid. This elaborate chemical process serves as the onion's way of deterring pests and animals from eating it.