Futility Closet offers Raymond Smullyan's self-referential proof of the existence of Santa Claus:
A: Santa Claus exists, if I am not mistaken.
B: Well, of course Santa Claus exists if you are not mistaken!
A: So, I was right.
B: Yes.
A: So, I was not mistaken.
B: Yes.
A: Hence, Santa Claus exists.
