Many hundreds of years ago in a small Italian town, a merchant had the misfortune of owing a large sum of money to the moneylender. The moneylender, who was old and ugly, fancied the merchant's beautiful daughter so he proposed a bargain. He said he would forgo the merchant's debt if he could marry the daughter. Both the merchant and his daughter were horrified by the proposal.

The moneylender told them that he would put a black pebble and a white pebble into an empty bag. The girl would then have to pick one pebble from the bag. If she picked the black pebble, she would become the moneylender's...