Why Onions Make You Cry

The story

Somewhere in a kitchen near you, a grown-up is chopping an onion. And even though nothing sad has happened, their eyes are filling with tears.

Why? Because the onion is fighting back.

An onion cannot run away. It cannot bite. It just sits there, a soft, juicy package of food, growing in the ground. Lots of hungry creatures would love to eat something like that. So onions carry an invisible trick: chemistry.

Inside an onion, packed into its cells, are some special ingredients. They are kept carefully APART, in separate little compartments, like things stored in different boxes. Sitting in their boxes, they mind their own business. But the moment a knife slices through the onion, the boxes break open, the ingredients meet, and a lightning-fast chain of kitchen chemistry builds something new: a stinging gas, far too small to see.

That gas floats up from the chopping board, drifts through the air, and lands on the wettest, most delicate thing nearby: your eyes. There it turns into a tiny sting. Your eyes cannot fight the sting, but they know exactly how to wash things away. So they do what eyes always do with an invader. They flood. Tears well up and roll down your cheeks, rinsing the stinging stuff away. You are not crying because you are sad. Your eyes are simply washing themselves, and doing a very good job.

Think about how clever that is for the onion. Underground, a nibbling creature bites in and gets a faceful of sting, and scientists suspect the onion’s sting helps put some nibblers off, though that is hard to prove underground. The onion has a burglar alarm made of chemistry, and it only switches on when something breaks in.

Now, can you outsmart an onion? You can, if you know how the trick works. The stinging gas is made when the insides mix, so anything that slows that mixing down, or catches the gas on its way, might help. Many cooks chill the onion first, hoping the cold slows the chemistry, though careful tests have not shown that one clearly works. Use a sharp knife: a sharp blade slices cleanly through, breaking fewer of those little compartments than a blunt blade that squashes and crushes.

And here is the happy ending: cooking changes those fierce chemicals into something completely different, which is why fried onions stop stinging and start smelling delicious.

So watch closely the next time an onion is being chopped at your house. Those tears on the cook's cheeks are not sadness. They are the end of a small kitchen drama: a vegetable answering the knife with an invisible spray, and a pair of human eyes calmly washing the sting away.