A commercial product used by hikers and skiers as a hand warmer, a heat pack consists of a plastic pouch filled with a supercooled solution of sodium acetate (NaC2H3O2).

When a metal disc in the pouch is flexed the solution crystallizes and becomes quite hot. The crystallization front is beautiful to watch. The pack may be reused many times by heating it in hot water to melt the solution. Eventually, however, it becomes impossible to keep the solution supercooled.