These little handwarmers contain a gel and a small metal disc. When you want to activate the handwarmer, just flex the disc a couple of times and the gel becomes hard, and will give off heat for up to 20 minutes.
We understand that the gel consists of a supersaturated solution of food grade Sodium Acetate. When you flex the metal disc, you create nucleation sites, that allow crystals to start forming. One theory is that flexing the metal disc releases one or more tiny particles of metal from the surface, and this tiny fragment acts as a starting point for the process of crystallisation. This process happens very quickly, and you see the crystals spreading across the hand warmer in a matter of seconds. This process gives off heat (it is exothermic) and so the whole hand warmer becomes hot, and stays like this for up to 20 minutes.
You can reactivate the hand warmer by placing it in boiling water for 5 - 10 minutes, until all the crystals have dissolved.
There are two handwarmers in a pack. They are 7.5cm in diameter.