That's what they call a bubble :)
The money they lend you, never really existed, was just numbers on a computer screen. But when you pay them back, it has to be reall money, which they are going to lend to other people eventually. But since it's just numbers on a computer screen, they will lend the same money to thousands of people. Till the day everything comes tumbling down. 2007 was a small example of what will happen.