Why does chopping onions make us cry?

Anyone who has ever chopped an onion has probably experienced the burning and tearing you get from the vapors. When your knife slices through a crisp onion, it tears through cells, releasing the cells’ irritating contents. Enzymes that were kept separate are now free to mix with the sulfonic acids to produce propanethiol S-oxide, a volatile sulfur compound that wafts upward toward your eyes. This gas reacts with the water in your tears to form sulfuric acid. As the nerve endings of your eyes’ corneas detect the irritant, your brain tells your eyes to produce tears to wash it away. In other words, all that crying is your body’s way of protecting your eyes.

Apart from wearing eye goggles or contacts there are other ways to protect your eyes such as refrigerating your onion before cutting it (slows reactions and changes the chemistry inside the onion) or by cutting the onion under water. Happy eating!
 

The reason that we cry while chopping onions, according to the Library of Congress, is because of the unstable chemicals that are released into the air during the chopping process.

The culprit that causes the tears is an enzyme called lachrymatory-factor synthase. When an onion is cut, the enzyme is released and converts the amino acids in the onion into sulfenic acid. That acid then transforms into syn-ropanthethial-S-oxide, which drifts into the cook's face and irritates the lachrymal glands in the eyes, prompting the production of reflex tears.