What is in the  Repeat the heat pack? 
All of the Repeat the heat packs contain water and food grade sodium acetate (a form of salt). The metal disc inside is made of stainless steel and the outer shell is medical grade plastic. 
How is the heat produced? 
By flexing (not breaking or cracking in half) the stainless steel disc, small molecules turn into a solid state which then creates a chain reaction causing the solution to crystallize and instantly heat to a predetermined amount of energy, 130° degrees F / 53° degrees C to be exact. The ability to create heat for a long period of time is accomplished due to the liquid NOT crystallizing at the same time. Molecules that were not required to heat the pack to its maximum temperature will remain liquid and crystallize as required, thus maintaining the maximum temperature. 
How long does the heat last? Does it lose heat? 

Repeat the Heat click and heat packs retain their heat for a period of 30 minutes and up to an hour. As your body draws heat from the heat pack the temperature will decrease gradually. All Repeat the Heat click and heat packs will maintain appropriate heat for therapeutic or doctor recommended timing, usually 15-20 minutes per use. 
Can 130° degrees F burn you? 
No…Per numerous health sites 158 degrees F is considered the burning point. Repeat the Heat click and heat packs are controlled to NOT exceed 130 degrees F, thus giving you perfectly controlled therapeutic heat, unlike other heat packs that are heated via microwave, conventional oven or uncontrolled. If you should notice sensitivity when using Repeat the Heat click and heat packs, please wait 15-20 minutes before placing on desired body location. 

Can I re-use my Repeat the Heat click and heat packs? 
Absolutely…Once the pack has given off all of its heat it will remain in a crystallized harden form until it is recharged. To regenerate your click and heat pack, place it in already boiling water and continue boiling for a period of 25-30 minutes (do not allow the vinyl coating to touch the pot or bowl directly), or until all of the crystals have melted back to a clear liquid state. You may notice bubbles, this is normal, carefully remove the pack with tongs from the water (watching you do not burn yourself do not pick up directly with your hand) and place on a cooling rack and let cool to room temperature before reactivating. To speed the cooling process, place the click and packs in the refrigerator. NOTE: The click and heat pack will not reactivate if its temperature is above 130° degrees F, cool time is mandatory for reuse.