Why Onions Make You Cry When Cutting

There are two chemicals inside an onion that never meet as long as the onion is intact — like two reactive substances kept in two separate jars.

When we cut into the onion with a knife, the cells rupture and those two chemicals finally meet and mix. The result is a gas that rises up toward your face. When that gas reaches the wet surface of your eye, it dissolves into your tears and forms a mildly acidic compound that makes your eyes sting. And because the eye stings, the brain runs its usual fix: it tells the eye to release tears and wash it out. So the crying is literally your eye's automatic rinse system doing its job properly.

So how do you stop your eyes from burning while working with onions?

Chill the onion first (15–30 minutes in the fridge): the cold slows the enzyme down and makes the gas more sluggish, so less of it rises toward you. This is the most effective trick.

Airflow: work under the range hood, next to a window, or with a small fan that carries the gas away so it never reaches your face.

A sharp knife: a sharp knife slices the cells, a dull knife crushes them. Crushing means more ruptured cells, which means more gas.