The story
A man travelling in a hot-air balloon realises that he is lost. He spots a pedestrian below and calls out:
“Excuse me, can you tell me where I am?”
The man on the ground thinks for a moment and replies:
“Yes. You are in a hot-air balloon, approximately thirty metres above the ground.”
The balloonist calls back:
“You must be a mathematician!”
“Correct. How did you know?”
The balloonist answers:
“Your information is exact, logically impeccable, and completely useless for my problem.”
Why the joke works
The punchline rests on the distinction between formal correctness and practical relevance. A statement can be true while still missing the actual question.
In mathematics, computer science, statistics and science, this is a useful piece of self-irony: precision alone is not enough. A good answer must understand the problem, respect the context and help the person asking.
The small epistemic lesson: “Correct” is not automatically the same as “useful”.