An adventurer found a locked treasure chest in a dungeon.
He tracked down three brothers (each either a Liar or a Knight) one of whom has the key to open the chest.
Each made a statement:
A: I have the key to the chest.
B: I don't have the key to the chest.
C: B Doesn't have the key.
The adventurer knows that at least one of the three is a Liar, and at least one - a Knight. Who has the key?
If A had the key, then all the three adventurers spoke truthfully. This is a conttradiction since there must be at least one Liar.
If B had the key, then all the three adventurers spoke falsely. This is a conttradiction since there must be at least one Knight.
Hence C must have the key.
Consequently, C is the Knight as he spoke truthfully, B is the Knight as his statement is obviously true, and A is the Liar having lied about the identity of the owner of the key.