A few years ago I received some heating pads. Little pads with a chemical stuff in it, SodiumAcetat. For use it's liquid and with a metal ring in it. If you bend that ring you start a chemical reaction and that substance is getting massive. It produces heath and that heath can be used to warm up your hands or pocket. You can reactivate that stuff by putting them in boiled water. I did use them sometime in the winter. Was fun. Gave them away after a while.
See the chemical section.
https://en.wikipedia.org/wiki/Heating_pad
Google on "heating pad SodiumAcetat". Compare the prices on Amazon and Alibaba. The latter are more realistic.
They're often used for advertisements/relation gifts.
George