Riddle: A prisoner is told, “If you tell a lie we will hang you; if you tell the truth we will shoot you.”
What can he say to save himself?
Answer: You will hang me. If they hang him, the statement was true and they could hang him only because he was lying.
If they shoot him, they make this statement a lie and shoot him just because he is telling the truth. A different solution, saying “You won’t shoot me” will cause a similar deadlock for murderers.