If you're a current or prospective investor in Bank of America (NYSE: BAC), one of the most important things you should probably know is how the nation's second-biggest bank by assets makes money. The answer to this question seems obvious, but there's more to it than meets the eye.To start out with, as a bank, Bank of America takes deposits and lends out money, pocketing the difference between its cost of funds and its yield on earning assets.