How do onions make your eyes water?

It is due to breathing through your nose and the associated odor of the onion. By breathing through your mouth (or gnaw on a toothpick?) you should be able to control the weepy eyes and nose. According to the food channel, it's because when you cut the onion, you pierce the cells which releases a chemical which irritates the pink tissue of the eyes & nose. Which are proximate to the source when looking down to slice & dice. Wanna real treat? Cut up some horseradish, it's guaranteed to run you out of the kitchen right fast!