Onions contain a variety of sulfur-containing compounds , one of which is the amino acid sulfoxide . When the onion is cut , an enzyme called allinase is released inside the cell , which converts the amino acid sulfoxides compounds into volatile sulfonic acid ; Which produces a compound called syn-propanethial-S-oxide , which comes in contact with tears , is responsible for bringing tears to the eyes. Simply put , contact with tears produces mild sulfuric acid , so it burns the eyes. Note that the sensation of burning sensation is detected by the brain through the free nerve ending above the cornea ( where many nerves come together) , then this sensation is transmitted through the ciliary nerve to the parasympathetic nerve lacrim . ( lacrimal gland) - stimulates. As a result, tears fall from the eyes.