Metals are valuable materials that can be recycled over and over again without compromising their properties. Scrap metal has a value that motivates people to collect for sale to recycling operations. Besides a financial incentive, there is also an environmental imperative. While recycling metals preserves natural resources, it requires less energy than manufacturing new products using virgin raw materials. Recycling emits less carbon dioxide and other harmful gases. Metals can be classified as ferrous or non-ferrous. Ferrous metals are combinations of carbon and iron. Some common ferrous metals include carbon steel, alloy steel, wrought iron, and cast iron. On the other hand, non-ferrous metals include aluminum, copper, lead, zinc, and tin. Precious metals do not contain iron. The most common precious metals include gold, platinum, silver, iridium, and palladium.