Great magicians seem to be able to break the rules of chemistry. They can pour water into a cup, and then second later turn the cup over to reveal that the water has magically turned into ice.
There's no real freezing involved in this process. Instead, there's a sponge at the bottom of the cup absorbing all the water - and the ice is added at the same time the water is. Having a sponge that's the same color as your cup makes this trick a lot easier.