Onion tears are caused by 2 different chemicals held apart by the onion's cellular structure. Every time you cut an onion, the cells rupture and mix, aerosolization and making you cry when the acid gets into your eyes. A sharp knife (as mentioning in another comment) would help minimize the amount of cellular damage and therefor the gas produced. You can also wear goggles.
Everything else is bunk. From-A former professional chef and person particularly sensitive to onions. (I just learned to cut onions with my eyes closed)