Why do onions make us cry?

You are making dinner, you start cutting an onion and, a few seconds later, tears are running down your face. It happens to almost everyone. But why does a vegetable make us cry?

The answer is a clever piece of chemistry. An onion is made of thousands of tiny cells. When you cut it, you break these cells, and some substances inside them mix together. This produces a gas that floats up into the air – and into your eyes.

When the gas touches your eyes, it irritates them. Your eyes react in the only way they can: they make tears to wash the gas away. So the tears are not a sign of sadness. They are your body’s way of protecting itself.

Scientists think onions developed this trick to defend themselves. In the ground, insects and animals that try to eat an onion get the same unpleasant surprise.

Luckily, there are some easy ways to cry less. Many cooks put onions in the fridge for half an hour before cutting them, because cold slows the chemical reaction down. A sharp knife also helps, as it breaks fewer cells than a blunt one. Some people cut onions near an open window or under a kitchen fan, so the gas moves away from their face.

And if none of that works? You could try wearing swimming goggles. You may look a little strange, but your eyes will stay dry – and your guests will never know.