of its metal or paper content. One justification for fiat money comes from a micro-founded model. In most economic models, agents are intrinsically happier when they have more money. In a model by Lagos and Wright, fiat money doesn't have an intrinsic worth but agents get more of the goods they want when they trade assuming fiat money is valuable. Fiat money's value is created internally by the community and, at equilibrium, makes otherwise infeasible trades possible. Another mathematical model that explains the value of fiat money comes from game theory. In a game where agents produce and trade objects, there can be multiple Nash equilibria where agents settle on stable behavior. In a model by Kiyotaki and Wright, an object with no intrinsic worth can have value during trade in one (or more) of the Nash Equilibria. History China China has a long history with paper money, beginning in the 7th century CE. During the 11th century, the government established a monopoly on its issuance, and about the end of the 12th century, convertibility was suspended. The use of such money became widespread during the subsequent Yuan and Ming dynasties. The Song Dynasty in China was the first to issue paper money, jiaozi, about the 10th century CE. Although the notes were valued at a certain exchange rate for gold, silver, or silk, conversion was never allowed in practice. The notes were initially to be redeemed after three years' service, to be replaced by new notes for a 3% service charge, but, as more of them were printed without notes being retired, inflation became evident. The government made several attempts to maintain the value of the paper money by demanding taxes partly in currency and making other laws, but the damage had been done, and the notes became disfavored. The succeeding Yuan Dynasty was the first dynasty of China to use paper currency as the predominant circulating medium. The founder of the Yuan Dynasty, Kublai Khan, issued paper money known as Jiaochao during his reign. The original notes during the Yuan Dynasty were restricted in area and duration as in the Song Dynasty. During the 13th century, Marco Polo described the fiat money of the Yuan Dynasty in his book The Travels of Marco Polo. Europe Washington Irving records an emergency use of paper money by the Spanish for a siege during the Conquest of Granada (1482–1492). In 1661,