A few days ago, we made a bottle rocket for a scout project. My wife is the scout leader, and they are working on a science badge. She asked me to make a bottle rocket, to show how air and water pressure works. It was fun to build, and really didn't take very long at all. I just used some pvc pipe, along with the right fittings.
All i needed to do then was make sure we had a good seal, and hook it up to a tire pump. We started out using a foot pump but pretty quick we went to a little electric pump. Fill the 2 litre bottle about 2/3rds full of water, and set in place then pump it up.
It was pretty impressive, it shot the bottle about 70 yards into the air. In fact if you look at these pictures the bottle landed in my yard next to that old blue pickup of mine. It was pretty fun, and the scouts loved it.