The concept of earning interest on money in the bank is so deeply ingrained into economic life that few people even know that the opposite can happen too: Banks can take a percentage of cash from your account in the form of negative interest rates, under certain conditions. Normally, this doesn't happen. Banks want your cash, and pay you interest on it, because the more deposits they have, the more they can lend it to others who pay them even more on their investments. But interest rates in Europe are so close to zero — and economic activity is so...