There are many sources of renewable fuel sources. The most common forms are wind power, hydropower, solar energy, geothermal power, and biomass. Wind power uses wind to turn turbines to generate electricity or pump water. A disadvantage is that it requires extensive areal coverage to produce enough energy. Hydropower uses water to turn turbines (like wind power) to generate electricity. Solar energy relies on the nuclear fusion power from the core of the sun to generate energy. Geothermal power is energy left over from the original forming of the planet and supplemented by heat from radioactive decay. Biomass is energy from plants. Its most popular form is burning trees for warmth.