Why onion makes you cry?!

Onion is one of the oldest vegetables known to human being.It’s an abundant source of nutrients, including vitamins B, C and G, protein, starch as well as compounds which have been said to be effective agents against fungal and bacterial growth; protecting against various cancers. In addition to that, it is used in a wide array of food preparations and recipes to add a specific flavor. Unless you've avoided cooking, you've probably cut up an onion and experienced the burning and tearing you get from the vapors. When you cut an onion, you break cells, releasing their contents. Amino acid sulfoxides form sulfenic acids. 

A sulfenic acid is an organosulfur compound and oxoacid with the general formula RSOH, where R ≠ H. It is the first member of the family of organosulfur oxoacids, which also include sulfinic acids and sulfonic acids, RSO2H and RSO3H, respectively. Sulfenic acids are produced by the enzymatic decomposition of alliin and related compounds following tissue damage to garlic, onions, and other plants of the Allium genus.

Cooking the onion inactivates the enzyme, so while the smell of cooked onions may be strong, it doesn't burn your eyes. Aside from wearing safety goggles or running a fan, you can keep from crying by refrigerating your onion before cutting it (slows reactions and changes the chemistry inside the onion) or by cutting the onion under water. The sulfur-containing compounds also leave a characteristic odor on your fingers. You may be able to remove or reduce some of the smell by wiping.

Or just a simple solution is to keep your head as far away from the onion as possible.

Onion possesses some medicinal properties, offering several health benefits. Onion is found to be effective to treat the medical conditions such as cold, diabetes, heart disease, osteoporosis and other diseases. They contain the chemicals that are believed to have anti-cholesterol, anti-inflammatory, antioxidants and anticancer properties.