Why do onions make cry?

Onions are particularly good at absorbing sulfur, which they use in a number of amino acids. When you cut an onion, you break open cell after cell, releasing their liquidy contents. Previously separated enzymes meet the sulfur-rich amino acids and form unstable sulfenic acids, which then rearrange into a chemical known as syn-propanethial-S-oxide. This sneaky chemical is volatile, meaning it easily vaporizes, and causes a burning sensation when it floats up from the cutting board and comes in contact with your eyeballs. In an attempt to keep your precious eyes from being damaged, your brain quickly triggers a tear response to rinse the offending irritant away. 

 Certain varieties of onions, especially sweet onions, are less sulfurous than others and are thus less likely to burn your eyes. You can also freeze or refrigerate onions before you chop them; this slows down the enzymes responsible for the series of unfortunate chemical events. Other tricks—such as lighting a match, eating bread while you chop, or breathing through your mouth—are considerably less effective.