If you’ve ever wondered why cutting into an onion makes you weep like a baby, here’s your answer. But fair warning, it’s about to get real sciencey up in here. Onion tears are a result of reflective tears, which is when smoke, dust or any other substance irritates the eye. You’ve probably noticed that when you cut an onion, your eyes start to burn before you cry. The eyes’ natural response to this burning is to produce tears to try and flush out the foreign substance.
When you cut open an onion, what you’re actually doing is breaking cells and causing a chemical reaction. The onion releases Propanethiol S-oxide gas that mixes with an enzyme in the onion to make a sulfur gas. They create a mild acid when they reach your eyes and cause you to cry. The gas and enzyme are kept apart when the onion is intact, but as soon as you break it open, they are released and mix together.
To have a less emotional experience with your onion, try chilling it in the fridge or freezer because as we all know from middle school science class, molecules move slower when they’re at colder temperatures. The gasses will release at a slower rate from the onion after being chilled and you’ll have more time before you start to bawl your eyes out.
Other tricks include cutting the onion under water to prevent the sulfur from reaching your eyes, breathing through your mouth, and dipping your knife in lemon or lime juice to mask the smell. But if none of these are working for you, you can never go wrong with a trusty pair of goggles. They’re the only thing guaranteed to keep the gasses away from your eyes.