Don't act like you're tough and all. You know you cry like a banshee when you start peeling them onions... uhm, providing you can do some kind of decent cooking of course. But don't you just hate that? I know I do because I end up having to stop peeling the onions and go find me a hankie or kleenex or something to dry my tears otherwise I'll end up chopping my fingers along with the onions since I can't see a damn thing with all those tears in my eyes!
When you cut an onion, you break cells, releasing their contents. Amino acid sulfoxides form sulfenic acids. Enzymes that were kept separate now are free to mix with the sulfenic 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. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away.
Cooking the onion inactivates the enzyme, so while the smell of cooked onions may be strong, it doesn't burn your eyes. Aside from wearing safety goggles or running a fan, you can keep from crying by refrigerating your onion before cutting it (slows reactions and changes the chemistry inside the onion) or by cutting the onion under water.
The sulfur-containing compounds also leave a characteristic odor on your fingers. You may be able to remove or reduce some of the smell by reading my question - How Do I Get Rid of the Onion Smell From My Hands?