Alberto and Benito are two brothers very similar in appearance but very different in character. It is known that one of the two is a compulsive liar while the other is a great man who always tells the truth.
Physically they seem so similar that I am unable to distinguish them, so I approach one of them and ask him: Is Benito a liar? and he answers: Yes.
Who am i talking to?
Without a doubt I am talking to Alberto since whether it is the liar brother or the one who always tells the truth, he will never be able to tell himself that he is lying because if it were the liar brother he would be telling the truth and if it were instead the brother who always Tell the truth I would be lying. Therefore, the one who answered could not be Benito himself. What we cannot know is which of the two brothers is the liar.