If you hate chopping onions because of the stinging, burning sensation and the tears, then you'll want to try this tip. I learned it a couple of years ago and have tried it out several times. I have found that it works beautifully.
The secret to tear free onion chopping is simply to breathe only through your mouth. The only trick is to remember to keep doing it the whole time you're chopping. If you have a hard time with that and you keep finding yourself switching back to breathing through your nose, then try chewing a piece of gum, chewing with your mouth open. The only time I will ever encourage that!