Three spies, suspected as double agents, speak as follows when questioned:
Albert: “Bertie is a mole.”
Bertie: “Cedric is a mole.”
Cedric: “Bertie is lying.”
Assuming that moles lie, other agents tell the truth, and there is just one mole among the three, who is the mole?
Answer
Bertie is the mole. Both Albert and Cedric are telling the truth.
When Albert said, “Bertie is a mole,” he was telling the truth, and giving the correct answer. When Bertie said, “Cedric is a mole,” he was lying, as he himself is a lying mole. When Cedric responded, “Bertie is lying,” he was telling the truth, and also affirming that Bertie was lying.