The most common reusable heat pads contain a supersaturated solution of sodium acetate in water. Crystallization is triggered by flexing a small flat disc of notched ferrous metal embedded in the liquid. Pressing the disc releases very tiny adhered crystals of sodium acetate into the solution which then act as nucleation sites for the crystallization of the sodium acetate into the hydrated salt (sodium acetate trihydrate, CH3COONa·3H2O). Because the liquid is supersaturated, this makes the solution crystallize suddenly, thereby releasing the energy of the crystal lattice. The use of the metal disc was invented in 1978.