Why onions make us cry and how to cut them

HostI was in the kitchen last night trying to make a big pot of chili, and within two minutes, I had to walk away because my eyes were stinging so bad. It feels like such a weird thing for a vegetable to do to you. What's actually going on when we cut into an onion?

GuestWell, you have to think about where the onion lives. It spends its whole life growing under the dirt. Down there, everything wants to eat it. Pigs, bugs, worms, you name it. So the onion has spent a long time building a way to fight back. It's basically a chemical weapon in the shape of a ball.

HostA chemical weapon? That feels a bit dramatic for something I put on a burger.

GuestBut it really is. It works like a booby trap. Inside the onion, there are these tiny little rooms, so small you can only see them with a magnifying glass. Some of those rooms hold one kind of stuff, and other rooms hold a different kind. As long as the onion is whole and sitting on your counter, those two things never meet. But the second your knife slices through, you smash those rooms open. The stuff spills out and mixes together.

HostSo the mixing is what causes the trouble? I always thought it was just a really strong smell that hit my nose and made me sneeze or something.

GuestNo, it's way more sneaky than a smell. When those things mix, they create a gas. That gas is very light, so it floats up from the cutting board and into the air. It's looking for water, and the wettest thing on your face is your eyes. When that gas touches the water that keeps your eyes wet, it has a change. It turns into a very weak version of acid. It's the same kind of stuff you find in car batteries, just way, way weaker.

HostWait, like acid? You're telling me there's acid forming on my eyeballs?

GuestJust a tiny, tiny bit. Not enough to do real damage, but plenty to make your brain feel the burn. Your brain feels that sting and thinks something bad is in there. So, it does the only thing it can do to clean it out. It opens the taps. It sends a flood of tears to wash that acid away. That's why the more you cry, the more it seems to sting for a bit, because the gas has more water to work with.

HostOkay, so if it's a gas, can I just wear a pair of swimming goggles? I mean, I would look like a total dork, but would it stop the crying?

GuestIt actually would. Anything that keeps the gas from touching your eyes will work. But most of us don't want to keep goggles in the kitchen drawer. Most people try these old tricks that don't really do much.

HostLike the bread thing! My grandmother always said if you hold a piece of bread in your teeth while you chop, the bread soaks up the gas before it hits your eyes.

GuestYeah, I have heard that one, and the one about keeping a match in your mouth too. The truth is, those don't really work. The bread is too far away from where the gas is actually going. If you want to stop the tears, you have to change how you use the knife. Most people have dull knives in their kitchen. When a knife is dull, it doesn't really slice. It crushes.

HostI guess I never thought about it like that. It feels like it's slicing.

GuestIf you looked at it really closely, you would see it's smashing those tiny rooms I talked about. A sharp knife is like a thin needle. It slides between the rooms or cuts them so cleanly that less of the stuff leaks out. If you use a very sharp blade, you're letting off way less of that gas. That's the best way to save your eyes.

HostWhat about the fridge? I have a friend who swears by putting her onions in the freezer for ten minutes before she starts cooking.

GuestThat actually has some real science behind it. Heat makes things move fast. Cold makes things move slow. If the onion is cold, the stuff inside doesn't mix as quickly. The gas stays inside the onion longer instead of flying up into your face. So, a cold onion and a sharp knife together make a huge difference.

HostIt's wild that we have to do all this just to eat a root. Why do we even keep eating them if they fight us this hard?

GuestBecause the same stuff that makes the gas is what gives the onion its kick. When you cook an onion, those harsh things change. They turn sweet and savory. If we grew an onion that had no sulfur in it, it wouldn't make you cry, but it would also taste like nothing. It would just be a crunchy, watery ball. We put up with the sting because we want the flavor.

HostSo the pain is the price of the taste. I guess that makes sense. But I still feel like there has to be a better way than just cutting faster and hoping for the best.

GuestYou can also use a fan. If you have a small fan on your counter blowing across the cutting board, it pushes the gas away from your face and toward the other side of the room. It's all about moving that cloud of gas before it finds the water in your eyes.

HostIt's funny to think of it as a cloud. Like a tiny weather system right there on my counter.

GuestIt really is. And once you know how it works, you can beat it. You just have to be smarter than a vegetable that's trying to defend itself from a gopher.

HostIt's pretty impressive that a plant sitting in the dark for months can cook up a plan to make a human being cry just by being touched.

GuestWe keep searching for that perfect onion that won't sting, but the truth is, the thing that makes us cry is the same thing that makes the onion taste so good when it hits the hot pan.

HostThe very thing that hurts us is what makes the soup worth eating.