A coin from ancient Rome featuring a depiction of Brutus -- yes, the Brutus who killed Julius Caesar -- sold at auction for $2 million. It got us wondering, in 2,000 years, could a penny be worth just as much? And what's the future of physical currency look like anyway? To find out, we talked to Aaron Klein, a senior fellow at the Brookings Institution, about what our money could look like in the future.