Two engineers were riding in a hot air balloon. They were blown off course and were completely lost. They finally saw a guy on the ground and they shouted "Can you tell us where we are?" After a few minutes, the guy yelled back "You're up in a balloon." One engineer said to the other, "Just our luck to run into a mathematician". The other engineer responded, "How do you know he was a mathematician?" "Well, in the first place he took a long time to answer; second, his answer was 100% correct; and third, it was totally useless."