Simple Rocket Tutorial
The rocket consists of a 1 liter bottle with three fins and a tennis ball for recovery. The effort of construction is about one hour. The result will be a simple but powerful water rocket. Here is the tutorial. A German version is also available.
The rocket goes pretty high if you use enough pressure. Make sure not to endanger pedestrians or animals!
Never touch the rocket when it is pressurized! Do not launch it in a hurry or in case of any doubts!