At present, most of our energy comes from fossil fuels - oil, coal, and natural gas. However, such reserves are limited. Because power demand is increasing very rapidly, fossil fuels will be exhausted within a relatively short time. Therefore, people must develop and use alternative sources of energy.
One alternative source of energy is geothermal heat, which comes from deep inside the earth. Scientists use this heat to make energy. However, this energy is available only in a few places in the world.
The sun, water, and the wind are other alternative sources of energy. The wind turns windmills, and moves sailboats. It is a clean source of energy, and there is lots of it. Unfortunately, if the wind does not blow, there is no wind energy. When water moves from a high place to a lower place, it makes energy. This energy is used to create electricity. Water power gives energy without pollution. However, people have to build dams to use this energy. And as dams cost a lot of money; so water energy is expensive.