Synthetic materials come from natural resources that go through a chemical process. Paper begins as wood, whose fibers are cooked apart with chemicals or ground apart and then pressed into sheets. Many plastics begin as petroleum or natural gas, whose small molecules are joined into giant chain molecules called polymers. Glass begins as silica sand, which melts at very high temperature and cools into a clear solid that is not a crystal. Steel begins as iron ore, an iron oxide rock that is heated to free the iron and then mixed with a little carbon. Each material gets a job because of its properties, and making and throwing away materials changes the world around us.