You slam your hand in a door, and the experience becomes etched into your brain. You carry a memory of the swinging panel, the sound as it crushes your flesh and the shooting pain as your skin gives way. Your body remembers it too. For days afterwards, the neurons in your spine carry pain signals more easily form your hand to your brain. As a result, your hand feels more sensitive, and even the lightest touch will trigger an unpleasant reaction. It’s as if your spine carries a memory for pain.

This is more than a metaphor. Two groups of scientists have found that one special molecule underlies both processes. It helps to store memories in our brains, and it sensitises