Onions contain amino acid sulfoxides in them which forms sulfenic acids in the onion cells. The enzymes in the onion and the sulfenic acids are kept separate within the cells. When an onion is cut, the enzymes and the sulfenic acids start mixing and produce propanethiol S-oxide, which is a volatile sulphur compound. The gas that is emitted reacts with the water of your eyes and forms sulphuric acid. The sulphuric acid thus produced causes burning sensation in the eyes and turns on the tear secretory glands to produce tears to wash off this acid.