Cutting or chopping up onions with a knife causes damage to their cells. As a defense mechanism, the onions release an enzyme into the air. This form of sulfuric acid irritates the eyes thus giving us tears. To prevent this make sure that your knife is razor sharp to lessen cell damage to the onion. When chopping keep your mouth slightly open and breathe through there instead of your nose. Also keep the root part of the onion intact to minimize the enzyme spread into the air as most of it is at the base of the onion.