Pyramid selling: You give me one pound, so that I can "invest" it, and I promise to pay you back two pounds. I persuade two more people to give me a pound each, and then I give you the two pounds which I got from them. I persuade four more people to invest a pound, and I give those four pounds to the two people who gave me the pound each. Then I persuade eight more people.... etcetera. There are soon not enough people left, so the last lot of investers lose their money.