Super-cooled water is super fun the first time you see it. Still in liquid state, it's actually well below zero—so when you touch it with a spoon, say, it turns solid right before your eyes. The best bit? You can make it at home.
All you need to do is shove a few bottles of purified water—you can pick it up from the drug store—in your home freezer for at least two or three hours, never opening the door at any point. Then, when you've waited, you can carefully take the bottles out to play with. Try opening the lid and pouring it out onto an also-cooled tray, or hitting the bottle hard against a wall—you should see ice forming right before your eyes. Try it, it's fun. [Sci Show]