Have the kids make a simple water wheel from a circle of cardboard, small paper cups, a dowel and staples. Demonstrate the history of the water wheel, including raising water from river valleys to agricultural fields, grinding grain, panning for gold and even the mechanics behind early amusement park water rides.
Explain to the kids how early water wheels were horizontal and could only supply direct energy. In other words, any use of the power from the water was built directly on top of the water wheel. Demonstrate how horizontal water wheels worked and ask the kids why they think these early water wheels were abandoned for upright water wheels.
Discuss more complex models of water wheels to illustrate the value of indirect production of energy. Explain how this concept led to the development of the water turbine and later to such inventions as the automobile engine and its ability to turn the wheels on a car. Have the kids try to think of other examples of the concept.
Illustrate the concept of gravity with a simple water wheel. Place the water wheel in a flat trough half-full of water and ask the kids why it doesn't turn, or turns very slowly. Tip the trough and demonstrate how the water increases speed and the wheel turns faster. Use this project to explain the concept of gravity and why water runs downhill. Ask the class to think of other uses for this knowledge, such as building pipelines to bring water to their homes and to water fields of crops.