Cutting an onion ruptures its cells and releases enzymes that react with sulfur compounds stored inside, producing a volatile gas called syn-propanethial-S-oxide. That gas drifts up and reacts with the moisture in your eyes to form a mild sulfuric acid, which irritates the eye's surface and triggers tears as a protective flush. Chilling the onion first, cutting it underwater, or using a very sharp knife (which ruptures fewer cells) can all reduce how much gas is released.