A merchant owed a sum of money to a usurer.

The usurer was old and ugly, but he drooled over the merchant’s beautiful young daughter.
So he proposed: as long as he gets the merchant’s daughter, his debt will be canceled.
The usurer said, “Let’s leave it to divine will to decide this matter.”
He would place one black and one white pebble into an empty money bag,
and let the girl pick one. If she chose the black pebble, she would have to marry the usurer, and her father’s debt would be forgiven.
If she chose the white pebble, she wouldn’t have to marry him, and the debt would also be canceled.
But if she refused to pick, her father would be sent to prison.
At that moment, they were standing in the usurer’s backyard,
on a small path paved with black and white pebbles.
The usurer picked up two pebbles, and the sharp-eyed girl noticed that they were two identical black pebbles.
Then, the usurer asked the girl to pick one.
She reached into the money bag, took out a pebble,
and before anyone could see which one it was, she dropped it on the ground.
Since the ground was covered with black and white pebbles, it was impossible to tell which one had just been dropped.
Because the remaining pebble was definitely black, and the usurer dared not admit to his earlier deception,
the debt was canceled, and the girl didn’t have to marry the usurer.