When we cut onions, they make us produce tears. The onion has developed this defense to defend it from predators that might try to eat it! Each tiny cell of an onion has a sealed blob within it containing a chemical the onion makes. This chemical is called an enzyme. Enzymes are very useful, natural substances in all living things that make chemical reactions happen. When you cut an onion you are tearing the sealed blobs containing enzymes apart. The enzymes escape. Once they have escaped they start reacting with another substance naturally in the onion, called sulfoxide. The enzymes and the sulfoxide react to make a new substance called sulfenic acid. This sets off a chain of other reactions one after another. The result at the end of this chain of reactions is another new substance called syn-propanethial S-oxide. The syn-propanethial S-oxide is lighter than air, so it floats. These onion fumes go inside of your eyes and cause it to produce tears!