Borrowed from the back cover of the book, The Use of Lateral Thinking by Edward deBono:
Many years ago when a person who owed money could be thrown into jail, a merchant in London had the misfortune to owe a huge sum to a money-lender. The money-lender who was old and ugly fancied the merchant's beautiful teenage daughter. He proposed that they let Providence settle the debt, and told them that he would put a black pebble and a white pebble into a money-bag and then let the girl pick out one of the pebbles. If she chose the black pebble she would become his wife and her father's debt would be cancelled. If she chose the white pebble she would stay with her father and the debt would still be cancelled. But if she refused to take a pebble her father would be thrown into jail.
Reluctantly the merchant agreed. They were standing on a pebble-strewn path in the merchant's garden as they talked and the money-lender stooped down to pick up the two pebbles. As he picked up the pebbles the girl, sharp-eyed with fright, noticed that he picked up two black pebbles and put them into the money-bag.
What did the girl do to ensure that she would not have to marry the money-lender and that her father would not have to pay the debt?