Humans have been trying to figure out the easiest way to pay for things for a very long time. First, they traded goats and shells, then gold and coins, then they got fancy and upgraded to cardboard Diners Club cards—followed, eventually, by plastic ones. Now cash and plastic are fading in popularity, and possibly faster than you realize. Maybe you already pay for your morning coffee with just your mobile phone. Soon enough, thanks to advances in biometrics, you might pay with just your face. Well before the end of the century, wallets will likely be museum pieces. 　　Futurists sometimes joke that the prospect of a cashless society is much like that of a flying car: it’s often promised, but it never materializes. And yet some banks in Sweden no longer dispense cash. Most airlines won’t accept cash for in-flight purchases. A number of restaurants, including New York’s Commerce, have begun refusing greenbacks, accepting only credit and debit cards.