To demonstrate the third law of motion.
Water rocket with water, as photographed.
ONLY used in large Lecture Halls. BE CAREFUL where you aim the rocket!
Air is compressed in the rocket by means of a pump; when the air is released, the rocket rises by a small amount. If a small amount of water is poured into the air compartment and air compressed in the rocket to the same pressure as before, the rocket will rise very high when released. Due to its greater mass, the water exhaust has more momentum than the air; thus more reaction force is applied to the rocket by the exhausting water.
(PIRA 1N22.20) See Demonstration Reference File for a number of articles on various aspects of rocketry.