Two men moving in a balloon are lost in the desert. They see an individual meditating in the shade of a tree.
- Where are we, please? they ask him.
After a long moment of reflection, the man answers them:
- In a balloon.
- Thank you, Mr. Mathematician.
The man asks astonished:
- How did you know I was a mathematician?
- For three reasons, answer the aeronauts. First, you thought a lot before answering us. Second, your answer is very accurate. Third, it is useless.