When we cut onions, we break cells, releasing enzymes that irritate our eyes. Water helps avoid tears when cutting onions. Put some water in your mouth and let it hang out while chopping. Do not swallow the water, until you finish cutting. Also you can cut onions in the sink under running water.
Use a very sharp knife while cutting onions. A sharp knife will cause less damage to an onion, and fewer irritants will be released.
A tip from the Chef: A sip of white wine will prevent tears when chopping onions.