We've probably all seen something like this by now - you take an empty 2 litre plastic soda bottle and a bucket of water, you put a piece of perforated foil over the top, cut the botto... ah, no. Wait. Different technique for getting into space. You use a bicycle pump type thing, and make a rocket out of it.
Discovery Channel's best program, Mythbusters, recently made one, and found it to be more powerful than they had anticipated. No shit. Take a look at what they do with them in Japan.
I am so building one of these things.