Sherlock Holmes and Doctor Watson flying hot air balloon. They get lost, so they have to land. They have no idea where they are.
Sherlock Holmes asks passing by guy: "Excuse me sir, but where are we ?" Guy says: "You are in hot air balloon" and he walks away.<p>After some thinking Sherlock says : "That person was a mathematician". "How did you know that ?" asks Doctor Watson. 
Sherlock Holmes replies: "Well first he gave a very precise answer, and second - the answer was completely useless"