As we go into winter, it’s time to start pulling scarves and fluffy coats out the wardrobe, and keeping hand warmers in our pockets. If, like me, you like getting value for money, you might consider getting a reusable hand warmer instead of a one-time use one. They work quickly and effectively: simply snap the metal plate and wait for the crystals to grow and your hands to warm.

But how does it work? A wonder of chemistry, clearly. But some keywords are saturated (well, actually supersaturated), instability, crystalisation and exothermic. You can probably piece together a rough idea of what happens, so below I’m going to go into more depth and detail!

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