As we cut the onion, a tear-stimulating substance is released. This substance is called lachrymatory factor synthase. During cutting, this material mixes with the amino acids in the onion and reacts to form sulfinic acid. To complicate matters a little more, sulfinic acid spontaneously converts to syn propanethial S-oxide, which is already a very volatile substance and is therefore easily released into the air, and when it reaches the eyes, it irritates the corneas of the cornea, which then triggers easy excretion. [img alt_text = 'Why are onions' description = ''] https:///wp-content/uploads/2017/01/csipos-hagyma.jpg [/ img]