One of our favorite questions from readers for Economics in Plain English was deceptively simple: What, after all, is money? And what sets it apart from something that's simply valuable? A big abstract idea like this called for a hands-on experiment.In this episode, business editor Derek Thompson pays a visit to a branch of EagleBank in Arlington, VA, to bother the world's friendliest bank teller with a series of dumb requests. As goofy as it seems, this little experiment is a helpful way to illustrate three essential functions of money: a store of value, a unit of account, and a medium of exchange. But you'll have to watch to see why.