We all know that onions make us tear up. The next time you are around someone who is cutting up an onion, impress them with your scientific knowledge.
Onions contain amino acid sulfoxides, which are a type of organic molecule. When you cut into the onion skin, allinases, a class of enzymes, mix with the sulfoxides to make sulfenic acid. Once exposed to oxygen in the air, the sulfenic acid arranges into the strong chemicals syn-propanethial-S-oxide and thiosulfinates, which cause tears and cause the strong smell of cut onion, respectively. It's not the scent of the onion that makes you cry, but rather the syn-propanethial-S-oxide irritating the eye that makes you release reflex tears and cry. Since the chemical makes you cry it's also called a lachrymator.