Why do onions make us cry? When you cut an onion, you break open its cells, releasing an enzyme called alliinase and amino acid sulfoxides. These react to form sulfenic acids, which are then reorganized by a second enzyme into syn-propanethial-S-oxide. This gas is volatile and drifts up toward your eyes. When it hits the water in your eyes, it turns into a mild sulfuric acid. Your brain then signals your tear ducts to produce more water to flush out the irritant. To prevent tears, many 'everyday' chefs recommend chilling the onion before cutting, which slows down the chemical reaction, or cutting the onion under water.