text: i have a question for a material or chemical engineer? i have a small warm pack for massages that is clear plastic with a clear fluid inside.  there is a metal disc inside that you flex back and forth to activate the heat.  this makes the clear liquid turn opaque and warms up.  to reactive, you boil the warm pack for 5 minutes until the inside fluid turns clear. what fluid is contained in this?  what does the flexing of the metal disc do?  thanks in advance! your heat pack probably contains sodium acetate.  flexing 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.  this 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.   when they pack is boiled, the crystals dissolve in their own water of crystallization so the heat pack returns back to a liquid state.  i hope this was helpful!