As luck would have it, two hot air balloonists out for a pleasant jaunt in the countryside are having some problems, and become irrevocably lost. Passing close to hill, they notice a solitary figure on top of it, and call down to him.
``Hello! Hello!'' call out the hapless ballonists to the man, ``Can you tell us where we are?''
This person-on-a-hill, after some small period of deep thought, responds ``You're in the air''.
One of the ballonists turns to his compatriot, and says ``You may rest assured that that man is a mathematician, for three reasons. First, he thought about the problem for an inordinantly long time. Second, his answer was absolutely precise. And lastly, it was completely useless''.