Of all the kitchen inconveniences the cook has to live with, the one that generates the highest number of defensive strategies is no doubt the chopping of onions, and the associated teargas effect.
The reason why it makes you cry is explained in detail here, and if you like to read about enzymes and syn-propanethial-S-oxide, as do I, it is worth a read.
But to put it more simply, chopping onions causes the release of an irritant gas in the air, which, upon reaching your eyes, triggers a blinking and tearing reflex designed to wash it away. Yet another illustration, albeit an annoying one, of what a nifty machine the ol’ human body is.
Such an unusual tip could not go untested, so I soon tried it, using the butt end of a loaf of pain au levain and feeling both experimental and silly, but I am happy and amazed to report it worked perfectly.
Not all onions are created equal (the fresher the onion, the less you cry) and not all cooks are as sensitive, but this phenomenon explains the volume of tips and tricks floating about — some of them amusingly contradictory — designed to either hinder the release of said gas, or prevent it from reaching the eyes.
Some people rinse the onions in cold water after peeling, or chop them underwater. Some recommend keeping onions in the fridge, or plopping them in the freezer for a few minutes before chopping. Some chop from the stem end down, others from the root end up. Some recommend breathing through the nose, others only through the mouth, while others still hold a sip of water in their mouth, and try not to laugh and spit it out.