Why Onions Make You Cry

Chopping onions is a common kitchen task. It often ends with burning eyes and tears. Why does this happen? The answer is simple chemistry.

An onion is a plant. While it grows underground, it absorbs sulfur from the soil. The onion uses this sulfur to make helpful molecules. When you keep the onion whole, these molecules stay safe inside its cells.

Problems start when you cut the onion. The sharp knife breaks the cell walls. This releases the sulfur molecules and special enzymes into the air. These two things mix together instantly.

The mix creates a new gas. This gas is volatile, meaning it floats up quickly into the air. When the gas reaches your face, it touches the water on the surface of your eyes.

The gas reacts with the water in your eyes and turns into a mild acid. Your eyes notice this acid immediately. To protect your eyes, your tear glands make lots of water. The tears wash the acid away, but they make you cry.

You can stop this process easily. Chill the onion in the fridge before cutting it. Cold slows down the chemical reaction. You can also use a very sharp knife. A sharp blade cuts cleanly and breaks fewer cells, so less gas escapes into the air.

Do you have a favorite trick to stop crying when you cut onions?