At present, most of our energy comes from fossil fuels (coal, oil and natural gas). However, reserves are limited. Because demand for electricity is rising rapidly, fossil fuels will be exhausted in a relatively short time. Therefore, people must develop and use alternative energy sources.
An alternative energy source is nuclear power. Nuclear power can provide enough electricity for the needs of the world for hundreds of years, but it can be very dangerous. An alternative source of geothermal energy comes from deep within the earth. Scientists use heat to generate energy. However, this energy is only available in some parts of the world.
Sun, water and wind are alternative sources of energy. Wind rotating the windmill and moving the sailboat. It's a clean source and a lot of energy. Unfortunately, if the wind does not blow, there is no wind energy. As water moves from one place to another, it makes energy. However, people have to build dams to use this energy source. The dams cost a lot of money, so water is expensive too.