Suppose somebody wants to make a bet with me that the San Francisco 49ers will win the next two Super Bowls. He gives me $100 today, and I have to give him $100 million in case he's right. The chances of this happening are very small, but just in case the impossible happens I want some backup. I buy insurance from my next-door neighbor. I offer to give him a nickel every week in return for his promise to cover my bet.
My neighbor sees that he has a good thing going -- getting money for nothing. After a while he takes on more and more bets until others follow in his footsteps. Soon, a market develops. In effect, people can bet on bets. Eventually, the total potential amount of money builds up into the billions and trillions of dollars.
Unexpectedly, the San Francisco 49ers win two Super Bowls in a row. My neighbor does not have $100 million on hand to cover my loss. The nickels I have been giving him have been wasted. I don't have $100 million either.
Suddenly everybody in the market is worried about people's ability to back up their bets. The Federal Reserve steps in and takes over the market. The free world is saved.