Sun, gravity and rocks – what do they have in common? They are the ultimate source of virtually all energy on earth. All fossil fuels, such as coal and oil; wood; and biofuels are organic compounds resulting from the capture of solar energy by plant photosynthesis. Solar energy also powers the air currents that we call "wind" and plays a major role in the hydrologic cycle. The natural decay of radioactive elements in rocks, primarily uranium, thorium and probably potassium, is the major source of heat at the earth's core (geothermal energy) and the nuclear fission or fusion of highly refined (concentrated) radioactive elements provides energy for power plants and our "atomic" weapons. The force of Earth's can be converted into hydropower by harnessing running water, while the gravitational pull of the moon provides a largely unexploited source of tidal energy.
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