Science Puzzle
Why Do Onions Make You Cry?
A whole onion sitting in the vegetable rack doesn't sting your eyes at all. You can hold it right up to your face and nothing happens. Slice into it, though, and within seconds your eyes are watering.
If the stinging chemical wasn't escaping from the whole onion, where does it suddenly come from?
The Answer
It isn't there until you cut. The onion makes the stinging chemical on the spot, the moment its cells are broken open.
Onion cells store harmless sulfur-containing compounds in one part of the cell and an enzyme, a chemical that speeds up reactions, in another. Like the two liquids sealed apart inside a glow stick, nothing happens while they're kept separate. A knife smashes the cells, the ingredients mix and the enzyme sets off a chain of reactions. The final product is a gas called syn-propanethial-S-oxide.
The gas drifts up and reaches your eyes, where it triggers the nerves that sense irritation. Your tear glands respond by flooding your eyes to wash it away.
Scientists think it's a defence: an animal that bites into an onion gets a nasty surprise. You can fight back by chilling the onion first, so the gas evaporates more slowly, and by using a sharp knife, which slices cleanly through cells instead of crushing them.
The principle: Chemical defence. Some plants store two harmless ingredients apart and mix them only when their cells are damaged, producing an irritant exactly when and where it's needed.