Wikipedia defines 'fungibility' as the property of a good or a commodity whose individual units are capable of mutual substitution, such as crude oil, wheat, precious metals or currencies. For example, if someone lends another person a $10 bill, it does not matter if they are given back the same $10 bill or a different one, since currency is fungible; if someone lends another person their car, however, they would not expect to be given back a different car, even of the same make and model, as cars are not fungible. Fungibility refers only to the equivalence of each unit of a commodity with other units of the same commodity. Fungibility does not describe or relate to any exchange of one commodity for some other, different commodity.