You know that moment when you’re cooking and then your eyes get itchy and tears are running down your face all of a sudden? No, I’m not talking of one of your existential crisis but actually of when you start cutting onions.
Just like other plants would have thorns, onions induce a chemical reaction to protect themselves from being eaten by any unwanted visitors. While growing, they absorb the sulfur present in the soil. So when you’re cutting it and breaking its cells, the sulfur enters in contact with another substance resulting in the creation of a gas. It is when that gas reaches your eyes that your lacrimal glands start producing tears to cleanse your eyes.
But rest assured, techniques exist to avoid crying! Some type of onions are less aggressive than others. You can try freezing or refrigerating them 30 minutes before cutting them. You can also wear a pair goggles. Or if you’re skilled enough, you can invent a new way to prepare them without breaking any cells. Of course these techniques work more or less, you just have to try them out!
-by Melainie
http://theconversation.com/why-do-onions-make-you-cry-129519