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Hi SeanyThis has been discussed a couple of times on the show. This first link is to the show where the kitchen science was about how they work.http://www.thenakedscientists.com/HTML/podcasts/2003-2004/show/2006.11.19/The text below is from an earlier posting....I'm pretty sure that what they work with is sodium acetate. What you've got is a super-saturated solution of sodium acetate, and as this begins to crystallise, it gives off a constant heat of about 55 degrees centigrade, which is a nice comfortable temperature for something like a hand warmer. Now how do you get this to work? Well as you bend the metal, I think you create surfaces that are like the crystals that could be formed from the solution (these are nucleation sites). Once you've done this, you're really seeding the crystals, and so one forms and then another and another until it spreads throughout the bulk of the hand warmer. All the time it is giving off this heat. Eventually, the heat will stop because you've crystallised the whole thing. You can repeat the whole process by putting the used hand warmer in something like and oven, until the crystal is all melted again.