Why onions make us cry

Why does a tiny onion make a grown-up cry?

Here is a plain onion, a round bulb with many layers, like a ball of papery wrapping.

Inside, each layer is packed with tiny cells, and the onion keeps two different chemicals locked in separate places.

As long as the onion is whole, the chemicals stay apart, and nothing happens. Now the knife comes down. It slices through the cells and breaks their walls open.

• Whole onion = chemicals kept apart

Now the two chemicals mix, like pouring two jars into one bowl, and they react together.

The reaction makes a stinging gas. Scientists call it the tear gas of the onion.

The gas floats up from the board and drifts straight toward your face.

Your eye is covered by a thin layer of water. The gas dissolves in it and makes a mild acid. That stings! Nerves in your eye shout danger to your brain.

• Gas + eye water = mild acid

Your brain answers by sending tears to wash the irritant away. Crying is your eye's cleaning crew.

So how do cooks cry less? Chill the onion first, or cut it near running water.

Cold slows the reaction, so less gas forms, and a sharp knife breaks fewer cells. Quick check before we finish. Cutting mixes the onion's chemicals into a stinging gas, and your eyes make tears to wash it away.

• Cold + sharp knife = less gas