Tears and onions? We usually get tears while cuting onions.why does it happen after all?onions contains Amino acid-sulfoxides,which converts into sulfenic acid in the onion cell.on cutting the onion,the enzyme starts 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 reactst 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 tear;-(;-( in order to dilute and remove the irritant.