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 in tandem with the enzymes in the onion to unleash a passive sulphur compound found within the onion.
The result: as you cut, the gas moves upwards and, combined with the water in your eyes creates sulphuric acid.
Your eyes aren't happy, even if you are, and react in the only way they know how when irritated by a foreign substance – they start tearing. Rubbing your eyes with your hands is about the worst way to alleviate the problem, since your hands are likely full of the tear-inducing agent too.
We've heard all kinds of folk remedies for onion tears, ranging from donning scuba diving gear to rubbing the onion with lemon, to wearing gloves & masks while you cut.
Leave a comment if you have any fancy remedy.