If you've ever used a heating pad or hand warmer, you essentially know what "hot ice" is. It's supersaturated sodium acetate, and it's actually fairly easy to make at home out of sodium acetate crystals. You can also make it out of vinegar and baking soda (directions at the bottom of this article).
Put the sodium acetate crystals into almost boiling water.
Keep adding sodium acetate and keep stirring until no more can be absorbed. You will need about 100 grams of sodium acetate for about 70 grams of water.
Put the solution in an airtight container and then put the container in the fridge.
When it is poured out into a dish it dries absolutely hard in the air.
It can also be used for sculpting by pouring it slowly so that it forms a column sticking up from the base.
Sodium acetate is not only supersaturated, but it's also supercooled. The freezing point of sodium acetate is 58°C, but this is at 18°C and isn't frozen, so therefore it's supercooled.
If you don't have sodium acetate crystals, you can also make this with vinegar and baking soda. Get 1 liter (33.8 ounces) of vinegar and 4 Tbsp of baking soda. Add the baking soda bit by bit so it doesn't explode. Then boil off 90% of the solution. Keep boiling it until a small crust forms. Then take it off the boil and cover in a water/air-tight container. Put it in the fridge for an hour and a half.
Once cool, you can take it out and touch it.