We learned that all the energy we use comes from the Earth. We make electricity using the Earth’s resources like coal or natural gas. Natural gas and coal are called “fossil fuels” because they were formed deep under the Earth during dinosaur times. The problem is that fossil fuels can't be replaced, when we use them up, they're gone forever. Another problem is that fossil fuels can cause pollution. We learned about alternative energy. Renewable energy is sustainable as it is obtained from sources that are inexhaustible (unlike fossil fuels). Renewable energy sources include wind, solar, biomass, geothermal and hydro, all of which occur naturally. Renewable energy is also called "clean energy" or "green power" because it doesn't pollute the air or the water. Unlike natural gas and coal, we can't store up wind and sunshine to use whenever we need to make more electricity. If the wind doesn't blow or the sun hides behind clouds, there wouldn't be enough power for everyone. Next we learned that it costs more to make electricity from wind and most people aren’t willing to pay more on their monthly utility bills. Then we looked at each renewable energy to understand how they form electricity.