Alan, Barry and Carl (each either a Knight or a Liar) were rounded up by the cops yesterday, because one of them was suspected of having stolen money from the bank. The three brothers made the following statements under very intensive questioning:
Alan: I'm innocent.
Barry: I'm innocent.
Carl: Barry is the guilty one.
If only one of these men is a Knight, who stole the money?
Since Barry and Carl disagree, one of them is a Knight and the other a Liar, but in any case Alan is a Liar, so he is guilty.