Hot hands are a reusable heat pack that can be used multiple times.
There is a metal disk inside the pack than when clicked activates an exothermic chemical reaction as the solution inside the package crystallizes and released heat.
After the crystals cool down they can be reactivated by hot water.
Most manufacturers recommend a crock pot, but I have also used a pot of boiling water. I have had success with, but cannot recommend, dropping the hot hand into a cup of water and after completely immersing it, putting it in the microwave.
I like this type of product much more than the iron oxide cloth heat packs that are commonly used. Those style are a single use only and the heat released is intense enough to burn you.
The reusable hot hands get hot, but it is not as intense, so it is much safer.
Besides, you can keep them around and use many times. I have had mine for over 3 years and have not had a single problem.
Now, the pack I got did not have detailed instructions so it took me a while to get the hand of the process.
The main thing to remember is heat it completely to melt not allowing ANY crystals to remain, and to let it cool completely before you try to activate.