When we chop an onion, we break cells of an onion, releasing their contents. A chemical process results, eventually releasing a compound (Amino acid sulfoxides form sulfenic acids) that causes you to tear up when you’re slicing and dicing.

Enzymes that were kept separate are now free to mix with the sulfenic acids to produce propanethiol S-oxide, a volatile sulfur compound that wafts upward toward your eyes. This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away.

How not to cry while chopping an Onion-

There are a few ways to do it.

·         Wear safety goggles or run a fan. This actually prevents the vapors from the compound entering your eyes—or at least blows the compound’s vapors safely away.

Refrigerate your onion before cutting it. Doing so slows reactions and changes the chemistry inside the onion. You can accomplish the same thing by cutting the onion under water.