We have three people one of whom is a knight, one a knave, and one a spy. The knight always tells the truth, the knave always lies, and the spy can either lie or tell the truth. The three persons are brought before a judge who wants to identify the spy.
A says: "I am not a spy."
B says: "I am a spy."
Now C is in fact the spy. The judge asks him: "Is B really a spy?"
Can C give an answer so that he doesn't convict himself as a spy?