In economics, a good is a material that satisfies human wants and provides utility, for example, to a consumer making a purchase while getting an enough-satisfying product. A common distinction is made between 'goods' that are tangible property, and services, which are non-physical. Commodities may be used as a synonym for economic goods but often refer to marketable raw materials and primary products. A good is a consumable item that is useful to people but scarce in relation to its demand, so that human effort is required to obtain it. In contrast, free goods, such as air, are naturally in abundant supply and need no conscious effort to obtain them. Although in economic theory all goods are considered tangible, in reality certain classes of goods, such as information, only take intangible forms.