Ice

From Wittypedia

If you put water in a freezer it goes all solid - that's ice. If you put a cat in a freezer it'll die - so don't do that. Ice has many purposes - such as providing a name and lyrics for the popular beat combo 'vanilla ice', as an item for putting down people's backs to annoy them and as a hideously ineffective lampshade. But the most common use is as a home for penguins This requires huge freezers which are evidently invisible - as no one has ever seen them and yet there's all this ice so it must be true.

Heat melts ice and reverts it to water - so if you are ever attacked by it when unarmed (without a hairdryer or such) your best form of action would be to rub your hands together vigorously and charge at it, screaming at the top of your lungs. The screaming has no effect and, to be honest, slightly warmed hands will take a very long time to desolidify the frozen assailant, so a well trained block of ice will still kill you... but it's better than sitting around waiting for the inevitable.