This document discusses different types of natural energy sources. It explains that the sun, wind, water, trees and fossil fuels are all natural sources of energy found in nature. It categorizes these sources as either renewable or nonrenewable - renewable sources like the sun, wind and water can be replenished within a human lifetime, while nonrenewable sources like fossil fuels take much longer to renew and should be conserved. The document provides examples and descriptions of how each energy source works and is used by humans.