Why Onions Make You Cry: The Science Behind Your Tears

Why do onions make you cry? It starts when you cut into the onion and damage its cells.

This damage allows substances that were stored separately inside the onion to mix. A series of chemical reactions then produces a volatile irritant called syn-propanethial-S-oxide.

Because it easily becomes airborne, this chemical can travel from the cutting board to your eyes. When it reaches the sensitive surface of your eyes, it irritates the nerves there.

Your tear glands respond by producing extra tears. These tears are a protective reaction that helps wash the irritant away.

Here is an interesting detail: the onion does not store this tear-causing chemical in its finished form. It is created after the onion's cells are damaged.

To reduce tears, try chilling the onion before cutting it. Using a sharp knife may also help because it can damage fewer cells than a dull knife.