Here is a question I was given during an interview a few years ago.
There was a farmer who had two daughters. One day he was considering giving his farm to his girls. He took his first daughter, Danielle, aside and told her in strictest confidence the area of the farm. He then took his second daughter, Tammy, aside and told her in strictest confidence the perimeter of the farm. He also told both girls together that the farm was a rectangle, that the perimeter of the farm did not exceed 80 miles and that each side of the farm was not less than two miles in length, and that all measurements were in whole miles. He then promised them that if they could tell him the dimensions of the farm without revealing to each other what he had told them in confidence, in that case he would give them the farm.
The conversation between the two girls went as follows:
Danielle said, "I don't know what the dimensions of the farm are."
Tammy responded, "I knew you were going to say that Danielle!"
Then Danielle said, "Well, I still don't know what the dimensions are!"
So Tammy said, "Well, I know what the dimensions are now!"
And Danielle concluded the conversation by saying, "Then I know what the dimensions are too!"
There was no other conversation between the girls. They went to their father, told him the dimensions of the farm, and their father kept his word and gave them the farm.
PS: Even if they do not know the value, each girl knows what information have been released to her sister. (Tammy knows that Danielle knows the area and Danielle knows that Tammy knows the perimeter.)