Bob, Joe and Sally have been traveling in a hot air balloon when it dawns upon them that they are lost. Joe looks down to the ground below for any indications of their whereabouts. Joe spots a man and yells down, “Excuse me sir, where are we?” The man ponders the question in silence. After a few minutes, the man looks up toward the balloon and shouts back, “You’re in a hot air balloon!” Bob turns to Joe and Sally, “Damn, that man was a mathematician.” “How do you know?” asks Sally. “Well” replies Bob, “He took a long time to answer our question, what he told us is absolutely correct and what he told us is absolutely useless.”