From wearing goggles to rubbing the knife with lemon juice, cooks have come up with a wide variety of tricks to try to keep themselves from crying while slicing onions. Scientists, however, already know why onions make us cry. When cut, they release a chemical that stimulates the nerves responsible for producing tears. The irritating compound goes by the name syn-propanethial-S-oxide. So, in order to help us avoid wearing goggles in the kitchen, a team of physicists decided to explore the underlying mechanisms at play when syn-propanethial-S-oxide gets released. They discovered that thinner, sharper blades produced fewer droplets that moved more slowly and with less energy. Meanwhile, the thicker, duller blades caused an explosion of high-speed particles that were sprayed out at up to an astonishing 141 feet per second. That’s because the dull blade initially bent the onion skin, which caused pressure to build up inside. When the blade finally sliced through, it released all that built-up energy and sent the unpleasant, tear-inducing juice flying. The duller blades produced as much as 40 times more particles than the sharper blades. Faster cutting speeds produced up to four times as many droplets as slower speeds. So, the best way to cut onions without your eyes stinging is to cut slowly with a thin, sharp knife. Problem solved.
