What are some ways to prevent myself from crying while chopping onions?
Sharpen your knife before cutting.
This is, by far, the most effective way to avoid crying while cutting onions. Using a sharp knife limits the damage to the cells of the onion, so fewer gases get released. Basically, you're making cleaner cuts, so even if you finely chop the onion, you probably won't start weeping.
However, you need a really, really sharp knife for this to work effectively. Ideally, you'll also add in some of the tips from the other users—the functional ones, that is.
Cut your onion with the root on, turning it regularly to keep your eyes clear of the danger zone. Work in a well-ventilated area, and if you've got an oven fan, use it. Try not to position your eyes directly over the onion, and if it's appropriate to the recipe, quickly rinse off the onion slices when you're through with your cutting.
I'll leave you with one last tip for avoiding tears: Try not to form an emotional bond with your onion.
Cut your onions differently.
The onion's enzymes need to contact amino acid sulfoxides to create the acid. Those amino acid sulfoxides are found within the onion, but not uniformly; you'll find more around the inner bulb.
Therefore, you can greatly reduce your onion gas exposure by cutting a bit more carefully. To minimize waste, you can cut the onion into halves lengthwise, keeping the root intact, then thinly slice perpendicular to the root, taking care not to cut all the way through the root itself. You can also simply remove the root and the inner bulb before you start cutting, but this method's a bit wasteful.
Cut your onions underwater.
This method will stop 100 percent of the noxious gases from reaching your eyes. It will also prevent you from chopping quickly, and it might be dangerous, since you're handling a knife underwater.
You could simply run some water and cut the onion under it, but that's a pretty messy way to work. There's a reason that you don't see Michelin-star chefs plunging their hands into a vat of water every time they need to make a soup. You're better off simply rinsing your onions at the end of your prep.
Microwave your onions.
This sort of sounds scientific, since "microwave" is right there at the beginning of the sentence. The idea is that it rids the onion of some of its gases before you start cutting.
Spray your cutting board with vinegar.
This method actually has some scientific basis, as vinegar is a weak acid. By spritzing your cutting board with a bit of white vinegar (or any other vinegar, for that matter), you can slow down the chemical reaction.
Unfortunately, while this method is somewhat effective, it has a pretty substantial side effect: It makes your onions smell like vinegar. Depending on the dish, that might not be such a good thing, and some people may find the vinegar smell to be more repugnant than the onion gas.
Another method recommends covering your knife with lemon juice, which is a bit more pleasant. Again, it works, but it does flavor your onions.
Hold a wooden spoon between your teeth.
I'm not sure where this one came from, but there's not really any science behind it. While the wood could conceivably absorb some of the acidic agents rising up from your cutting board, it would only be slightly more effective than a metal or plastic spoon (in other words, it wouldn't really be effective at all).
But this did get me thinking; what about the cutting board itself? Could that hold a key?
This is the most obvious solution, and, predictably, it works. Airtight goggles should prevent the onion's enzyme from contacting your tear ducts. Of course, eye wear isn't always a practical solution.
Instead of breaking out the goggles, you might simply increase ventilation by opening a window or turning on a kitchen fan. You can also cut near an open flame, and the convection will suck away some of the gas. While this can be somewhat effective, it still doesn't address the underlying chemical issue.
Why do onions make us cry?
Cut into an onion, and you'll likely shed a few tears. That's due to a chemical reaction that acts as a sort of defense mechanism for the onion—but with the right method, you can avoid the waterworks.
First, a quick look at the aforementioned chemical reaction. Cutting an onion damages its cells, releasing an enzyme known as Lachrymatory-factor synthase. As that enzyme spreads, it converts amino acid sulfoxides into sulfenic acid, which stabilizes as syn-ropanethial-S-oxide. (Don't worry, this isn't going to be on the test.)
Syn-ropanethial-S-oxide acts as a lachrymatory agent, and we swear that's the last big word we're going to throw at you. When the agent contacts your eyes, it mixes with your tears to create sulfuric acid, which your eyes interpret as contamination (since, well, it is contamination).
To address the problem, your tear ducts go to work, creating enough tears to wash out the chemical. That doesn't immediately make things better, so you may find yourself crying for quite a while.
By understanding that chemical reaction, we can prevent it.