When you slice an onion, you break open its cells, causing two substances to mix: an enzyme called alliinase and sulfoxide molecules. This reaction produces a volatile gas called syn-propanethial-S-oxide. When this gas reaches your eyes, it reacts with the moisture to form a very mild version of sulfuric acid. Your brain detects this irritant and triggers the tear glands to produce water to wash it away. To prevent this everyday kitchen struggle, experts suggest chilling the onion before cutting (which slows the chemical reaction) or wearing goggles to block the gas entirely.