White and Black Pebble

Many years ago in a small town, a farmer had the misfortune 
of owing a large amount of money to the moneylender. 

The moneylender, who was old and ugly, fancied the farmer's 
beautiful daughter so he proposed a bargain. He said he would 
forgot the farmer's debt if he could marry her daughter. 
Both the farmer 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 wife and her father's 
debt would be forgiven. If she picked the white pebble she 
need not marry him and her father's debt would still be forgiven. 
But if she refused to pick a pebble, her father would be thrown into jail.

They were standing on a pebble strewn path in the farmer's garden. 
As they talked, the moneylender bent over to pick up two pebbles. 
As he picked them up, 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.
What would you have done if you were the girl? If you had to
 advise her, what would you have told her?

Careful analysis would produce three possibilities:
1. The girl should refuse to take a pebble.
2. The girl should show that there were two black pebbles 
in the bag and expose the moneylender as a cheat.
3. The girl should pick a black pebble and sacrifice herself 
in order to save her father from his debt and imprisonment.

The girl put her hand into the moneybag and drew out a pebble. 
Without looking at it, she fumbled and let it fall onto the pebble-
strewn path where it immediately became lost among 
all the other pebbles.

"Oh, how clumsy of me," she said. "But never mind, if you 
look into the bag for the one that is left, you will be able to 
tell which pebble I picked."
Since the remaining pebble is black, it must be assumed 
that she had picked the white one. And since the moneylender 
dared not admit his dishonesty, the girl changed what seemed 
an impossible situation into an advantageous one.
This story is to spot the​ difference between lateral thinking and 
logical thinking 
originally quoted by a well known authorEdward de Bono.