The unstable supersaturated solution of sodium acetate trihydrate crystallizes instantly upon contact with seed crystals (nucleation site). The colourless supersaturated solution was poured very slowly to create the stalagmite shape changing into a white solid. The crystallization reaction is exothermic, releasing energy in the form of heat (53-56 degrees celcius) to the surroundings. Not surprisingly, this same reaction occurs in commercial hand warmers, where a metal disc usually starts the reaction. Once it cools downs, it can be boiled in water to dissolve the crystals and use it again.