Stinging eyes and tears, caused by sulfuric molecules released when the cell walls of an onion are ruptured, are nature’s way of saying, “Back away!” It’s a deterrent for certain animals. (Onions are poisonous to cats and dogs—and only a little less hazardous to human first dates.) But those sulfur compounds also give onions the pungent flavor that we love. So first make sure that your knife is sharp—a cleaner cut means fewer ruptured cells—and follow these steps to slice with less suffering.
Chill the onion. Placing it in the freezer or a bowl of ice water for at least 30 minutes, or in the refrigerator for a couple of hours, may temporarily quiet the compounds.
After cutting the onion in half, rinse both sides under cold running water for 1 minute. This will wash away some of the irritants.