Oh, it's an old joke. It goes like this:

A man in a balloon realises he's lost and, spotting another man walking his dog in a field, descends to ask the dog walker for directions.

"Do you know where I am?" asks the balloon man.

"Yes" answers the dog walker. "You are in the basket of a hot air balloon, hovering a few feet over an empty field, talking to a man walking his dog".

"Ah", says the balloon man. "You must be a mathematician".

"That's right" says the dog walker. "How did you know?"

"Because" says the balloon man "you gave me an answer that is at once entirely correct and completely useless".