We're not talking about that little puddle of water on your kitchen counter that used to be an ice cube. Rather, "hot ice" is a substance called sodium acetate trihydrate. Solid at room temperature, it melts into a liquid at 58 degrees Celsius.
It can also be melted into a liquid and then supercooled, getting it down below its freezing point without having it crystallize. Then, when some disturbance point -- or a nucleus for crystals to form around -- is introduced, the rest of the liquid rushes to solidify as well.
In this video from NurdRage, adding nucleation sites -- in this case, a sodium acetate coating on the hands -- rapidly causes the jarful of molten hot ice to begin crystallizing.
It's warm and has the consistency of ice cream!
Sodium acetate and water are key ingredients in heat pads and hand warmers. Bending the metal disk starts the crystallization process, giving off heat as it solidifies. The packs can be reused by dissolving all the crystals in boiling water. But each and every single crystals has to be dissolved, otherwise it'll re-solidify.