Personal possession of currency is prohibited in U.S. prisons, although, in practice, prisoners still exchange goods and services. U.S. prisoners used cigarettes as a medium of exchange until smoking was banned in U.S. prisons in 2003. After 2003, prisoners looked to other items on the commissary menu to facilitate exchange. In 2008, the Wall Street Journal reported that energy bars had caught on as money in some U.S. prisons. One important difference between using energy bars and using dollars as money is that _________ have intrinsic value.U.S dollars are an example of ________ money.