Geoff Palmer, age 10, of San Francisco, California, for his question:

How do dams make electricity?

The cleanest power plants are run by waterfalls that do nothing to pollute the atmosphere. The mighty force of the falling water pushes a great generator around and around. The turning generator spins a great arm of copper coils and a mighty magnet around each other. This is what sends electricity zooming out through the wires. The dashing water turns the generator that generates the electric power.

A dam is built across a river to make the water back up and form a reservoir. Some of the water stored in the reservoir is used to supply homes and cities. Some is led away to water the farmlands. And some is allowed to dash out through narrow channels. This dashing water has the same forceful push as a waterfall. It is used to spin generators that send electric power through wires that can carry it hundreds of miles.