Actually it does expire. Not all of it, true. Some money lasts decades or more.<p>Paper money doesn't remain usable as long as metal coinage, but even some metal coinage is no longer accepted.<p>As inflation rears its ugly head, the smaller denominations become discontinued and no longer accepted by vendors. In some countries, whole bunches of coins are redesigned, leaving the old coins no longer acceptable. Sometimes whole types of currency are discontinued, leaving holders of that older currency (think Euros displacing French Francs) with lots of worthless paper. For that matter, think of holders of Confederate Notes in the US. Items of interest maybe, but not usable if you want to buy a Big Mac.