Did you know.....
My eyes burn, and water when i cut onions. A friend of mine told me to stick my head in the freezer when this happens and it really works!!!! I'm not sure if this is the proper area to post this, but i had to pass it on!
...for the rest of us, Cook's Illustrated found that using ski goggles was the only sure method to eliminate "crying".
Multiple other home remedies include burning a candle near the cutting surfact (burns the odor before it can hit your eyes), fill your mouth with water, and (my personal favorite which works for me), stuff you mouth with a wad of bread while cutting...
When you slice through an onion, you break open a number of the onion's cells which contain (among other things) certain enzymes. The enzymes escape and trigger a series of chemical reactions creating other compounds. Some of these newly created substances--amino acid sulfoxides--form sulfenic acids, which then quickly rearrange themselves into a volatile (though not harmful) gas.
The gas reaches your eyes and reacts with the water that keeps them moist. This changes the chemical's form again, producing, among other things, a mild sulfuric acid, which irritates the eyes. The nerve endings in your eyes are very sensitive and so they pick up on this irritation (this is why one's eyes may sting when slicing onions). The brain reacts by telling your tear ducts to produce more water, to dilute the irritating acid so the eyes are protected. You may also rub your eyes, making the irritation a lot worse, especially if you have onion juices all over your hands.
Oddly enough, this volatile compound is also responsible for a lot of the great taste in onions, as well as the pleasant aroma when you cook the vegetable. You'll also get sulfenic acids by cutting up garlic, chives and leeks, among other vegetables, but they don't form the same irritating gas, just a strong smell.
Peeling the onion and then chilling it in the refrigerator or freezer before you slice it will minimize the release of gas somewhat, because the change in temperature alters the compounds in the onion. Cooking an onion before you slice it will work also, for the same reason.
Some people say if you hold a lemon, piece of bread or a sugar cube in your mouth, the food will absorb the gas before it reaches your eyes. Breathing with your mouth, instead of your nose, might also help because as you inhale, you suck the gas in and as you exhale, you blow it away. This keeps a lot of the gas from ever reaching your eyes.
What works for me is to keep my mouth shut- don't open it all while cutting onions and there are no tears. This is probably a variation of the holding objects in your mouth, but the key is that the mouth is shut while holding the lemon, piece of bread, etc.