прикол старый, но до сих пор не удавалось найти описания.
тут в каментах народ делится опытом http://en.wikipedia.org/wiki/Sodium_acetate
Sodium acetate is also used in consumer heating pads or hand warmers and is also used in "hot ice". When sodium acetate trihydrate crystals (melting point 58 °C) are heated to around 100 °C, they melt. When this melt cools, it gives a supersaturated solution of sodium acetate in water. This solution is capable of supercooling to room temperature, well below its melting point, without forming crystals. By clicking on a metal disc in the heating pad, a nucleation center is formed which causes the solution to crystallize into solid sodium acetate trihydrate again. The bond-forming process of crystallization is exothermic, hence heat is emitted.
ну и напоследок: http://www.alhimik.ru/read/olg21.html