Someone asked about these:
HEAT WAVE Instant Reusable Hand Warmers - 1 pair
These can be used hundreds of times, I've had ones for years and lost count of how many times I've used them over the years. This is what happens qhen you click the disc, and the pack heat up to 55 ºc or so, stays pretty warm for 30 minutes.
To turn it back to liquid, just boil it. I place a couple of metal spoons in the bottom so as to avoid direct contact with the bottom of the pot and risk burning the plastic. After a few minutes of boiling, when you see that all crystals disolved, just let it cool and its ready for another use by clicking the disc.