Tears flow from the eyes while cutting onions, but do you know what the scientific secret behind it is? Let's understand it in a short and interesting way. Onion's self-defense system Onions produce a special chemical to defend themselves to avoid animals and insects. When we cut onions, their cells break, and the enzymes and sulfur compounds inside combine to form a gas, which is scientifically called syn-propanethyl s-oxide. This gas causes irritation in our eyes. Effect on the eyes This gas reaches the eyes through the air and dissolves in the thin, moist membrane of the eyes (tear layer). This produces a mild sulfuric acid, which causes irritation. The eyes immediately activate their defense system to avoid this irritation and the tear glands produce more tears to wash away this harmful substance. Why do tears flow? These tears are actually the eyes' natural defense, released to protect them from irritation or damage, just like when dust or a particle gets into the eye. So, the next time you're cutting onions, don't let your eyes water. If you do, remember that this is your eyes' natural way of protecting themselves!