text: Your heat pack probably contains Sodium Acetate.\n\nFlexing a metal "trigger" within the sealed container causes a few molecules of liquid to crystallize.  These few molecule start a whole chain reaction which starts a chain reaction causing the supercooled solution to change from a liquid to a solid as crystals form.\n\nThis phase change causes the pack to give out heat. When the heat pack contents crystallize, its temperature returns its freezing point, and then you can recycle it by boiling. \n\nWhen they pack is boiled, the crystals dissolve in their own water of crystallization so the heat pack returns back to a liquid state.\n\nI hope this was helpful!