Cutting an onion produces a series of chemical reactions that unleashes propanethial S-oxide, an irritant that causes the eyes to fill with tears. If you want to prevent tears, you have to chop in a way that produces less irritant, and prevents the irritant from reaching your eyes.
Here are our five favorite ways to stop the waterworks:
1. Use a sharp knife. A sharp knife will cause less damage to cell walls, and fewer irritants will be released.
2. Keep the exposed cuts away from you. As soon as you cut an onion in half, turn both halves down on your cutting board. Leave the side you aren't currently chopping unpeeled. Once you've finished with one half, move the diced onion into a prep bowl, and set it on the opposite side of the kitchen, before proceeding with the rest of the onion.
3. Cut the onion properly. Chefs have an efficient way of dicing onions which conveniently keeps the most of the exposed cuts against the board.
4. Chill the onions. With a cold onion, less propanethial S-oxide will evaporate. You'll still get some irritants, but this will help. Remember – refrigerate, don't freeze. Frozen raw onions are often mushy when they thaw.
5. Run the vent hood. Position your cutting board next to the stove and turn on the exhaust fan. Irritants will be pulled away from you and your eyes.