Q. Why do we usually get teras while cutting onions?
Ans. Onions contain amino acid- sulfoxides, which converts into sulfenic acids in the onion cells. On cutting the onion, the enzymes start to mix with the sulfenic acid to form propanethiol S-oxide gas, which is a volatile gas, that reaches our eyes. This volatile gas then reacts with the water of the eyes, forming mild sulphuric acid; causing a burning sensation in our eyes. This burning sensation stimulates the tear glands to secrete tears in order to dilute and remove the irritant.