Today's lesson in money comes courtesy of the comic strip Frazz.
One of the fundamental characteristics of money is homogeneity. That essentially means each monetary unit is the same as every other unit. They are interchangeable. This concept is sometimes hard for some people to understand - especially the very young. They often view banks as warehouses. They may believe that if you deposit five $1 bills in your account, the teller takes those five bills and puts them in a drawer with your name on it. When you withdraw five dollars, the teller will give you the same five bills.
Those of us with more experience know that isn't true. That your five dollar bills are intermixed with others and they circulate. The chance of receiving bills is very, very remote.
But because money is homogenous, it helps to make a fractional reserve banking system possible. As long as everyone doesn't demand their money at the same time, money can be lent. Those wanting to withdraw funds can be given any cash on hand.
But when too much is lent and there's a demand for funds, a fractional reserve system can become illiquid. That's one reason for a central bank. The discussion can go much farther from here, but the lesson in the cartoon is that the money we put in is not necessarily the same money we take out. The deeper discussion may be why.