Why do onions make your eyes water when cut?

When you cut an onion, you rupture 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 upward 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 trigger the lacrimal glands to produce tears as a protective response to flush out the irritant. The more you cut, the more gas is released, and the more your eyes water. Chilling the onion beforehand slows the chemical reaction and reduces how much gas is released, which is why cold onions cause less tearing.