The earth's resources are commonly divided into renewable and nonrenewable resources. Some renewable resources are perpetual, meaning that they are not affected by human use, such as solar energy or wind energy. Other renewable resources are organic and inorganic materials that are replenished by physical and biogeochemical cycles. Examples of organic renewable resources are all plant and animal species that people use for food, building materials, drugs, leisure, and so on. Examples of inorganic renewable resources are water and oxygen, which are replenished in the hydrological and oxygen cycles, respectively. The main sources of renewable energy in the United States are biomass (wood and waste burned for fuel), hydroelectric power (energy produced from flowing water), geothermal sources (energy from heat sources in the earth's surface), solar (energy from the sun), and wind energy. Nonrenewable resources are those materials that are present in the earth in limited...