This is why cutting an onion makes you cry. It's because it's evil*
*Actually it's because of cutting an onion breaks its cells open, thus releasing enzymes that turns sulfenic acid into propanethiol S-oxide, a volatile sulfur compound that wafts up to your eye, react with the water in your tears to form sulfuric acid. Sulfuric acid (the chemical that disfigured Gotham district attorney Harvey Dent and turned him into Two-Face in the Batman comics- see? Comics are scientifically useful!) burns, so your eyes produce more tears to wash it away. In short, onions make you cry because it's evil.