Despite the fact that personal possession of currency is not allowed by U.S. prisons, in reality, incarcerated people still exchange goods and services. Until 2003, the year in which the U.S. government banned smoking in federal penitentiaries, cigarettes were the preferred medium of exchange among prisoners. A main difference between using cigarettes and using dollars as money is that ____ have intrinsic value. U.S. dollars are an example of ______ money.