When an onion is ruptured—such as by a knife—its cells break open and release irritating compounds that form a substance called propanethial sulfoxide, which is similar to sulfuric acid, says Dr. Irwin Goldman, associate professor of horticulture at the University of Wisconsin-Madison.

As the nerve endings of your eyes’ corneas detect the irritant, your brain tells your eyes to produce tears to wash it away. In other words, all that crying is your body’s way of protecting your eyes. (It also seems to help if you block the cornea by wearing goggles—or even contact lenses. Or you could try CHOW contributor Daniel Duane’s method.)

Using “gene silencing” technology, Dr. Colin Eady of Crop & Food Research in New Zealand recently developed a tearless onion by shutting down the gene in the vegetable that produces the irksome substance. It’s still in the development stage, however, and there’s no word yet on whether the new onion tastes better or worse than its weepy relative.

CHOW’s Nagging Question column appears every Friday.

See more articles