Suppose there are twin brothers; one which always tells the truth and one which always lies. What single yes/no question could you ask to either brother to figure out which one is which?
Since you only get to ask ONE question, and you don’t know if you’re getting the brother who always lies or the brother who always tells the truth, you have to ask, “Which path would your brother say is the correct one?”, and then you would take the opposite path. It’s a double negative. If you ask the brother who always tells LIES which path his brother would say is the correct one, he is going to LIE and point to the WRONG path, so you would have to take the other path. If you ask the brother who always tells the TRUTH which path his brother would say is the correct one, he’s going to point to the WRONG path, knowing that the brother who lies would say to go that way – and once again, YOU would take the OTHER path. This solution works no matter which of the twin brothers you encounter, and it is the only solution.
