"If you're so wise," he said to Nasruddin, "please tell me: where is the center of the universe?" Then he added by way of emphasis, "The exact center."
Nasruddin stroked his beard, looking thoughtful. He then walked over to his donkey and stared at him intently.
"The exact center of the universe," Nasruddin declared, "is under my donkey's right hind hoof."
"Impossible!" retorted the fortune-teller. "You must prove to me that you're right!"
"No," said Nasruddin, "you must prove to me that I'm wrong."
Inspired by: Mulla's Donkey and Other Friends by Mehdi Nakosteen
Notes: This story is on p. 21 of the book. This story is sometimes told in a much more elaborate form where Nasruddin is serving in Tamerlane's court and several philosophers come to challenge him, each with impossible questions like this one which he answers in terms of his donkey.