Many of us have cried over cutting onions. But why do chopping onions make you cry? When you cut an onion, you break its cells and release its contents. Sulfenic acids are formed from the released amino acid sulfoxides. Enzymes are now free to combine with sulfenic acids, producing propanethiol S-oxide, which wafts upward toward the eyes.
Reacting with the water in your tears, this gas forms sulfuric acid which burns, further stimulating your eyes to produce more tears to wash the irritant away.