Peeling, cutting or crushing an onion's tissue releases enzymes called allinases, which convert molecules within the onion to sulfenic acids. The sulfenic acids then rearrange to form a chemical that triggers the tears. So it is not the odour, as commonly believed, which causes crying. The front of the eye, the cornea, is densely populated with sensory fibres. The cornea also has autonomic motor fibres activate the lathat chrymal (tear) glands. These fibres pick up the chemical from onions as a burning sensation and tears are produced to wash away the irritant. As for avoiding this mechanism, chopping them in a breezy atmosphere, under water or in a closed container may help you avoid the chemical.