The energy can be drawn from any number of sources: nuclear fusion within stars; nuclear fission in entities similar to our nuclear-power plants; chemical reactions, which on Earth typically involve combining oxygen molecules in the air with carbon-based molecules stored over long eras in deposits of coal, oil, or natural gas; motions of fresh-water streams, atmospheric gases, and ocean waves and currents, which arise from the energy input from a nearby star; tidal movements of the oceans, the result of gravitational forces from the sun and moon; and the internal heat within a planet, the result of the decay of radioactive minerals beneath the surface. Our society exploits all of these energy sources (though the last two of them, tidal motions and geothermal energy, barely rank as significant contributors to the total) with varying degrees of efficiency and environmental impact.