I have to write about this! It's just too cool! Onion goggles! One of my housemates got them as a present from a thoughtful parent last Christmas and I have been making good use of them. They work amazingly well! You can get them from Amazon.com for $18.98 or probably find them at the nearest kitchen gadgetry store. Wow, they even come in pink! They also make a fashionable accessory.
Why do onions make us cry? Here is the answer from The Naked Scientist: "Onions are a member of the Allium family of vegetables that also includes leeks, shallots, chives, garlic and sharp scallion. The tears come from volatile oils containing substances called amino acid sulphoxides, which give the members of this family of vegetables their characteristic smells. So why do we cry ? Well, when you cut the onion you also release substances called allinases which break down the amino acid sulphoxides to something called sulphenic acid which then turns into something called syn-propanethial-S-oxide which irritates nerve fibres on the surface of the eye, triggering tear production. Because it takes time for this substance to form, this explains why we don't start to cry until at least 30 seconds into cutting up an onion."
Then, again, if you don't want to splurge on these, swimming goggles work just as well. But are not nearly as cool.