energy
Consider petroleum as an example. Oil companies explore for petroleum in areas where they think it might be. When they find it, they must determine how much is there. They must also know how hard it will be to get. If there's enough to make it worthwhile, they will decide to go for it. To extract petroleum, companies they must build huge rigs, like the one in Figure below . An oil rig drills deep into the ground and pumps the oil to the surface. The oil is then transported to a refinery. At the refinery, the oil is heated. It will then separate into different products, such as gasoline and motor oil. Finally, the oil products are transported to gas stations, stores, and industries. At every step, energy is used. For every five barrels of oil we use, it takes at least one barrel to get the oil.