Onions can be a real pain, especially when you have to chop a lot of them. The cause is that they contain extremely volatile molecules that float out into the air and hit our face and make us cry. There are several ways to avoid this. One crazy way is to hold a match in between your lips (this doesn't work sometimes). The more sure ways to avoid the onion cry include having a burning candle near the chopping block, so that the fire will burn the flying tear-causing molecules. You can also of course wear goggles. Then, to make less molecules escape from the onion you can chill it in a refrigerator before chopping it.