Onions obtain sulfur from the soil and turn it into four kinds of chemical ammunition, which are stored in cell fluids. A separate storage vacuole holds an enzyme trigger. When enzyme meets ammunition—after your Santoku cuts through it—the result is a volatile sulfur compound that floats into your eyes. The mixture of gas and tears produces a very mild but profoundly unpleasant triple threat of sulfuric acid, sulfur dioxide and hydrogen sulfide.
The Solution: Refrigerating an onion for 30 minutes or chilling it in ice water slows down the action of its trigger enzyme and saps some energy from the vegetable's volatile molecules. Says Barry Swanson: "It reduces the tendency for the sulfur compound to volatilize"—so you'll look less distraught as you prepare dinner.—James Norton