Water rockets are one of the classic science demonstrations for kids - exciting, a bit of danger and some interesting science. Make sure you have plenty of space - making this video, I managed to get the rocket to hit the road, a swing set (with no one in it, thankfully) and some trees. We moved to the middle of a cricket field as the rockets can really go a long way. You can buy water rocket kits at toy or sciencey-styled stores. You need to provide a drink bottle, a bike pump and some water.
The science is quite simple. Using the bike pump, you pump air into the bottle (which already contains some water), increasing the pressure of the air (storing potential energy) until the seal on the rocket bursts. The high pressure air forces water out the bottom of the rocket, which propels the rocket forward. This is an example of Newtown's third law of motion (commonly known by the expression: for every action, there is an equal and opposite reaction).