Water bottle rockets work by pressurizing a plastic bottle with air and using that compressed air to force water out of the spout. The bottle works like a rocket engine, increasing the thrust provided by the pressurized water as it flows through a constricting nozzle. The forces involved can propel the rocket up to 2,000 feet in the air, although most hobbyist rockets travel shorter distances.
To prepare a water bottle rocket, you need a plastic bottle partially filled with water and some sort of an air pump. A hand pump is capable of providing enough pressure for lift off, but serious hobbyists often opt for an air compressor. Two-liter soda bottles can generally handle up to 100 pounds per square inch of air pressure, but pressurizing them that high can risk damage and rupture. Once the rocket is pressurized and sealed with a quick-release valve, it is ready for launch. Once the valve opens, the high-pressure air tries to escape, pushing the heavier water ahead of it. Since water is more dense than air, this provides much greater force and lift than pressurized air alone. The rocket will expel water and receive thrust as long as its interior pressure is greater than the outside air. Once the pressures equalize, the rocket will lose power and fall to earth.