The chemical that causes your eyes to water when you cut onions is called propanethial S-oxide. It is a strong lachrymatory, meaning it causes your eyes to water. When it is exposed to air, it quickly degrades into other chemicals, including sulfuric acid. The sulfuric acid causes the stinging sensation, which adds to the lachrymatory effect. Interestingly, onions do not contain any propanethial S-oxide. Instead, onion cells contain a variety of sulfur based chemicals and an enzyme that causes them to react. When the cell is damaged, these chemicals mix, producing the propanethial S-oxide.

Chilling an onion before you cut it slows down the reactions that produce the propanethial S-oxide, greatly reducing the tears and stinging.