The dividing line between natural resources and man-made resources is not clear-cut. Hydro-electric energy is not a natural resource because people use turbines to convert the energy from moving water. Petroleum and iron ores are natural, but need work to make them into usable refined oil and steel. Atomic energy comes from metallic nuclear fuels, like fissionable uranium and plutonium, but natural rocks need technical work to make them into these nuclear fuels.