The Managing Director of a large company was trying to promote one of his Regional Sales Managers to become the new Sales Director. He had interviewed every one of the managers, and had reduced the field to the final three candidates.
All three were exceptional at their jobs, but the Managing Director was looking for the man or woman with something extra - someone with the edge that would set his company apart. And so he set them a puzzle.
He showed them all a piece of red chalk and a piece of green chalk and then told them to face away from the other two candidates. He then explained that he would put either a red cross or a green cross on each of their foreheads - they might have all the same colour or there might be different colours.
Once he had done this he told the candidates that on his command they should all turn around and raise their hand if they could see a green cross. Then, if any candidate should know the colour of their own cross they should tell the Managing Director the colour.
If that candidate could explain their reasoning they would become the new Sales Director.
Once he was satisfied that they all understood the instructions, he put a green cross on all three foreheads and told the candidates to turn around. Immediately all three hands went up, quickly followed by one candidate saying that she knew the colour of her cross to be green.
How did she know?
She reasoned that, as she could see two green crosses, if her cross was red each of the other two candidates would be able to see one green cross and one red cross. Assuming that they are both intelligent, either of them would quickly work out that the green cross the other candidate could see must be theirs, and say they knew that their cross was green.
As neither candidate has said this, she knew that they must be seeing two green crosses, and be unsure. With her superior reasoning she concluded that her cross must be green.