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

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. 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"