Chop onions in your food processor. Be sure not to over-process the onions, and wait a few seconds before opening to give the tiny gas droplets produced by cutting the onion a chance to settle.
Keep onions in the fridge, or put one in the freezer for a few minutes before cutting. Although onions don't have to be refrigerated, chilled onions produce fewer tears.
Wash it away
Cut the onion under cold running water to wash away the irritants before they can make it up to your eyes. This can be fairly tricky but those who've mastered it say it's the only way. Of course, then you have to dry them with kitchen towel.
Avoid the root
The National Onion Association says that you can reduce tearing when cutting onions by first cutting off the top, then peeling off the outer layers, leaving the root area intact. Why? The root end of the onion has the largest concentration of sulphuric compounds that make your eyes tear. This will help to some extent, but by cutting the root itself, it may get the eyes stinging.
Use eye protection
Wear goggles -- such as swim goggles or woodworking goggles -- to protect your eyes from the vapour. It looks a little silly, but it really works.
Try the bread trick
As if the goggles tip wasn't strange enough... Try holding a slice of bread (or any food) between your teeth while chopping. How could that help? The theory is that doing so tends to force you to breathe through your mouth, making you less likely to get the irritants up your nose, thereby being less likely to tear up. Worth a shot.
Consolidate cutting tasks
If you use onions frequently, chop up several at once and store them in an airtight container in your fridge. Use them as as needed, which will save you time and tears! This is really the best idea if onions are used on a regular basis. Top chefs have sub chefs who mainly chop onions for tomorrow's meal. Take note.
Why the tears?
Why does chopping onions make us cry?