There used to be a magical mirror, called the Mirror of Truth. If someone looked into it and said a lie, they would die immediately. In the land where it existed, everyone from kings to merchants was subject to its justice. One day Mulla Nasrudin came to this land and was brought before the Mirror of Truth to test his honesty. He declared, “I am telling a lie.” Nothing happened. “I think I broke your mirror,” said Mulla.
(I don’t remember where I read this, but I just found it in some old notes and thought I’d share it here.)