Riddle: One day an alchemist brought a king a bottle holding a strange liquid. The alchemist said, "This liquid I discovered will dissolve anything it touches." How did the king know that he was lying?
Answer: The king knew if what he said was true the bottle would get dissolved!
Riddle: A salesman sold a parrot to a man. The salesman said, "This parrot will repeat every single word it hears." The man bought the parrot, but he found it would not speak a single word. What the salesman said was absolutly true, however. How is this possible?