The energy from the sun is sometimes trapped and preserved in the remains of long-dead organisms. A fuel is anything that can be consumed and used as a source of energy. Biomass fuels are fuels that come from living (or once living) organisms. Examples of biomass fuels include wood, coal, oil, natural gas, and ethanol.

When plant and animal matter is buried under conditions that include extreme heat and pressure, they do not decompose. Therefore, the sun's energy that was stored in their tissues is still viable and usable. In fact, this is the major source of energy that runs our planet today! Fossil fuels - coal, oil, and natural gas - contain energy stored by plants that lived hundreds of millions of years ago, when the Earth was partly covered by swampy forests. When the plants died, they were buried by a layer of dirt and water. The heat and pressure from the top layers compressed the plant remains into what we today know as coal. Oil reserves, which are composed of microscopic animal and plant remains, formed in a similar fashion.