This hand size heat pack contains a supersaturated sodium acetate solution, which at room temperature is in a super cooled liquid state, since it normally freezes well above this at 54°C. By bending the stainless steel clicker disc, the crystallisation process begins and within a few seconds the entire heat pack has solidified. At the same time heat is released (an exothermic process) because the temperature jumps up to 54°C which is the temperature at which it changes to a solid (freezes). This energy release in the form of heat is because you are forming bonds between atoms to get crystals. Boiling this reusable heat pack in a pot of hot water until all of the crystals have dissolved, and then leaving it to slowly cool, will allow it to be reused many times. Size 140 x 85mm - clear liquid.