Almost everyone has shed a tear or two for them: onions. When we cut onions, we initiate a complex chemical process. The inside of the onion cell contains the sulfur-containing amino acid isoalliin, and the cell wall contains the enzyme alliinase. When cut, the cells are destroyed and the substances come into contact. The result of the reaction is propanethial-S-oxide, which irritates our mucous membranes.
