A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel – the engine of the rocket – is usually a used plastic soft drink bottle. The water is forced out by a pressurized gas, typically compressed air. The bottle is partly filled with water and sealed. The bottle is then pressurized with a gas, usually air compressed from a bicycle pump, air compressor, or cylinder up to 50 psi. The seal on the nozzle of the rocket is then released and rapid expulsion of water occurs at high speeds until the propellant has been used up and the air pressure inside the rocket drops to atmospheric pressure. The expulsion of the water thus can cause the rocket to leap a considerable distance into the air.