Why Cutting Onions Makes Your Eyes Water
Onions, along with many other plants in the Allium species (garlic is another popular one), absorb sulfur from the soil. When onions are chopped, cells within the onion break and release certain enzymes. These enzymes then react with the sulfur, creating amino acid sulfoxides. These, in turn, create the highly unstable syn-propanethial-S-oxide, which is a combination of sulfuric acid, sulfur dioxide, and hydrogen sulfide. When this substance, in a gaseous state, comes in contact with the moisture in your eye, it triggers a burning sensation via the ciliary nerve.
Tears in the eyes are regulated by the lachrymal gland, which is situated just above your eyelids. When the brain gets a message that there is an irritant in the eye, such as the above syn-propanethial-S-oxide, which gives a burning sensation, it then kicks the lachrymal glands into overdrive, trying to flush the irritant out of your eye(s) with tears.
Cooked onions won’t produce this same effect because the process of cooking the onion inactivates the enzymes needed to make the syn-propanethial-S-oxide. So you can safely chew the cooked onions without getting teary-eyed.