Popular Science shows you how to make cool ice sculptures from the liquid inside hand warmers… –
The mechanism behind this is supersaturation. Hot water dissolves much more sodium acetate than does room-temperature water. Keep adding sodium acetate to a pot of nearly boiling water until it stops dissolving, and you’ve saturated the water. Let the pot cool and it stays liquid, even though now it’s holding far more sodium acetate than it should at the lower temperature: It’s now supersaturated.
The excess sodium acetate should precipitate out as the water cools, turning back into solid crystals. But it won’t do so without a trigger. In most hand warmers, that comes from a small piece of metal inside, but even a mote of dust could set it off. Once the process starts, it doesn’t stop until it’s all solid. The substance is not really freezing–in fact, the rapid precipitation releases heat, hence the hand-warming.
HOW TO – Transform hand warmers in to liquid ice sculptures – Link.