Maybe it happened to you that during winter your hands were so cold that it was hard for you to even move them. One of the best ways to deal with this is by using Phase Change Materials.
When these materials touch your cold hands they will change phase from liquid to solid and transfer heat to your hands so they get warm. PCMs bags easily solve this problem. Under extreme cold temperatures, just by pushing a button, the PCMs inside these bags transform from liquid to solid and the latent heat stored is released; its transformation produces heat that can warm the hands for as long as 2 hours. In order to use these bags again, just place them in warm water so that the heating energy coming from the water recharges them.