Why does chopping onions make your eyes water?

When you cut an onion, you break open its cells, releasing enzymes that react with sulfur compounds stored in the onion. This reaction produces a volatile gas called syn-propanethial-S-oxide. When this gas drifts up and reaches your eyes, it dissolves in the moisture on your eye's surface and forms a mild sulfuric acid. Your eyes detect this irritant and your tear glands respond by producing extra tears to flush it away. The effect is stronger the more vigorously you chop, since more cells are ruptured and more gas is released. Chilling the onion beforehand slows the chemical reaction and reduces how much gas is produced, which is why cold onions cause less eye irritation.