1. A hand warmer contains sodium acetate, dissolved in water. The solution is ‘super-saturated’, which means it has been heated to dissolve more sodium acetate. The solution crystallises readily.
2. When the internal metal strip is bent, tiny bits of metal are released, which offer ‘nucleation sites’ for crystals to form.
3. As the crystals spread, the stored heat energy of the solution is released, heating the hand warmer up to 54°C – an exothermic reaction.
4. The hand warmer can be reset by boiling it in a pan of water to liquefy the crystals.
Get more fascinating Q&As from BBC Focus magazine by following @sciencefocusQA