This is something I've always wondered. What is it about onions that bring tears to our eyes when we cut them? After doing some research, I found that when you cut onions, you break cells in in the release vapors which reach our eyes. The contents of the onions membrane, which contains amino acid sulfides, are now free to mix and chemically react to create another chemical known as propanethiol S-oxide, which is a sulfur compound that wafts upward toward your eyes. When these fumes reach your eyes, they react with your tear ducts, causing your eyes to water. When you cook the onion, the fumes may seem like they are stronger because the smell is stronger, but actually the enzymes have been deactivated so your eyes will not burn.