Almost every cuisine uses onions, which means they reduce cooks to tears the whole world over. This perennial problem has given rise to a great deal of kitchen lore, designed to minimize your participation in the crying game when prepping onions. Some of these suggestions actually work, while others -- such as chewing gum while you cut -- have no effect.
This Means War
Onions don't have the luxury of running away from potential predators, relying instead on chemical weaponry to keep themselves from being eaten. Their protective sulfur compounds -- which, ironically, give onions culinary value from the human perspective -- are highly volatile, turning into a fine mist when the onion is cut. When that mist reaches your eyes, it irritates their sensitive membranes and prompts a flood of tears to wash away the aggressive intruder. Chewing a wad of gum does nothing to alter that dynamic.
Practical and Pragmatic
The oft-repeated advice about cutting onions under cold running water has some merit, because the water absorbs those sulfur compounds and keeps them from your eyes. Unfortunately, wielding a knife with wet hands means you're pretty likely to cut yourself, which might also bring a tear or two to your eye. Refrigerating the onion makes its sulfur compounds less volatile, which helps. Using a sharp, thin-bladed knife limits cell damage to the onion -- another positive. The best tactic of all is to protect your eyes with goggles or a swim mask, which looks silly but works admirably.