A water rocket works because of air pressure that is builds inside a container filled with water, usually a plastic soda bottle, which is released rapidly through an opening as it tries to escape. The reaction of a water rocket is an excellent example of Newton’s third law of motion, which states that forces exist in pairs.
To make a water rocket, fill a plastic bottle partially full of water. Air from a bicycle pump or an air compressor is then pumped into the bottle. The compressed air creates a bubble that rises inside the bottle through the water. The pressurized air at the top of the bottle forces the water out of the opening, and the bottle rises.