Whenever you starts cutting onions, they start conflicting with your lachrymal glands to give off tears because onions contains an unstable chemical known as syn-propanethial-S-oxide which is released into the air.
Chewing gum while cutting onions stops tears by pressurizing you to breathe through your mouth. This scatters the irritant so that a small amount could reach the lachrymal glands, preventing them from being irritated enough to release tears.
Chewing gum before or during cutting of onions regulates the production of saliva. Saliva actually absorbs the vapors emitted by cut onions, stops them from condensing on the eye and causing tears. It is really the best way to start chewing gum before starting to cut onions, and remember that breathing through mouth while cutting the onion is very beneficial.