Onions make people cry due to the release of a chemical irritant called syn-Propanethial-S-oxide. This chemical is produced when onions are cut, specifically when their cells are damaged by the knife. Inside the cells of an onion, there are enzymes and sulfur-containing compounds known as thiopropanolsulfonic acids. When an onion is cut, the enzyme alliinase is activated, which converts these compounds into sulfenic acids. These acids are unstable and quickly break down into syn-Propanethial-S-oxide.

This volatile compound is then released into the air and can easily reach the eyes. When it comes in contact with the tear ducts, it reacts with the water in the eyes to form sulfuric acid. This acid irritates the sensory nerves in the eyes, causing a burning sensation. In response, the body produces tears to dilute and flush out the irritant. This is why people often feel the need to cry when chopping onions.

Interestingly, not everyone reacts to onions in the same way. Some individuals may experience a stronger burning sensation than others, and some may not react at all. The intensity of the effect is influenced by genetic differences, the type of onion, and even individual sensitivity to sulfur compounds.

The cause of onion-induced tears is the release of syn-Propanethial-S-oxide, a sulfur compound that irritates the eyes and triggers tear production as a natural defense mechanism.