Here is a joke I invented (based on a famous one) and had mixed reaction.
A young mathematician comes to present to a famous mathematician his conjecture and ideas. "You are absolutely wrong," the famous mathematician dismissed the young one. Next enters another young mathematician and presents precisely the opposite conjecture. "You are absolutely wrong" replies the famous mathematician. The famous mathematician's wife interferes. "How could you tell both of them that they are wrong," she sais. "They have made completely opposite claims, one of them must be right!" "You are also wrong," replied the famous mathematician.