Why do onions make your eyes water when you cut them?

When you cut an onion, you break open its cells, which releases 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 tears to flush it away. The effect is stronger the more cells you break, which is why chopping produces more irritation than simply peeling. Chilling the onion beforehand slows the chemical reaction and reduces how much gas is released, which is a common trick to minimize eye watering.