Some hand warmers are reusable, as the reaction is reversible. These use the formation of crystals from a salt solution, an endothermic reaction. A metal disc inside the product is 'snapped', releasing small particles into a supersaturated salt solution. These begin the crystallisation, which spreads throughout the solution, releasing energy as it does. These do not work for as long, however the placed in boiling water to dissolve the crystals, and then used again