A merchant owed a large sum of money to a moneylender. The old, ugly moneylender fancied the merchant's beautiful daughter. He proposed that he would forget the debt if he could marry the merchant's daughter.
The merchant and his daughter were horrified. The cunning moneylender suggested that they let providence decide the matter. He told them that he would put a black pebble and a white pebble into an empty bag. The girl would have to pick a pebble from the bag.
If she picked the black pebble she would have to marry the moneylender and the debt would be forgotten. If she picked the white pebble, she need not marry him, and the debt would be forgotten. If she refused to pick a pebble, the merchant would have to go to jail.
The moneylender bent over and picked up two pebbles. The sharp-eyed girl noticed that he had picked up two black pebbles and put them into the bag. He then asked the girl to pick her pebble from the bag.
On first impression it would seem there were only these possibilities: 1) she refuse to take a pebble, 2) she show that there were two black pebbles exposing the cheating moneylender, 3) she pick a black pebble and sacrifice herself for her father's debt.
Before you read the answer, below, let us see if you can figure out a solution to this problem.
There is one.
The girl nerviously picked a pebble out of the bag, fumbling and dropped it on the ground so that it couldn't be known what color it was. She then said: "Oh I'm so foolish, but never mind, we can look in the bag and see the color of the pebble that is left and we will know what color my pebble was."
Don't think there is not a wise solution!
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