One Day An Alchemist Brought A King A Bottle Holding A Strange Liquid. The Alchemist Said
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!
Source: https://www.riddles.com/index.php/2008
