Why Do Onions Make Us Cry?

Why do we cry when we cut onion? Is it because onions hate us and they make us cry?

Let's look at it from the point of view of the onion. An onion is perfectly polite to us until we start hacking at it with a knife. Alas, the act of cutting enlivens a gas, propanethiol S-oxide, which works with enzymes to unleash a passive carbon compound found in onion. The result : as you cut, the gas moves upwards and, combines with water in your eyes, and creates sulphuric acid. Your eyes aren't happy, even if you are, and react in the only way they know when irritated by a foreign substance - they start tearing. Water produced by the tear ducts washes the acid formed to remove the foreign material.

So We cry(start tearing) to to flush the sulphuric acid formed in our eyes due to the propanethiol S-oxide gas which is produced as we cut the onions.