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Syn-Propanethial –S-Oxide is chemical in onions responsible for tears. During chopping an onion release an enzyme called Synthase this enzyme convert the amino acids sulfoxides into sulfenic acid. The Sulfenic acid is unstable chemical it convert into Syn-Propanethial –S-Oxide is an volatile chemical mix in air and irritate lachrymal glands. Due to irritation of lachrymal glands tears will release.