 A specific sulfenic acid, 1-propenesulfenic acid, formed when onions are cut, is rapidly rearranged by a second enzyme, called the lachrymatory factor synthase or LFS, giving "syn"-propanethial "S"-oxide.The gas diffuses through the air and, on contact with the eye, it stimulates sensory neurons creating a stinging, painful sensation.Tears are released from the tear glands to dilute and flush out the irritant.