The rupee slumped to a record low of 71 against the dollar on Friday. Exchange rates are constantly fluctuating, but what, exactly, causes a currency's value to rise and fall? Simply put, currencies fluctuate based on supply and demand. Most of the world's currencies are bought and sold based on flexible exchange rates, meaning their prices fluctuate based on the supply and demand in the foreign exchange market. A high demand for a currency or a shortage in its supply will cause an increase in price. A currency's supply and demand are tied to a number of intertwined factors, including the country's monetary policy, the rate of inflation, and political and economic conditions.