Albert Einstein once boarded a train from Philadelphia. The conductor came around to punch the tickets and said, “Ticket, please.” Einstein reached into his vest pocket for the ticket, but did not find it. ①He checked his jacket pocket. No ticket. He checked his brief case. But still, ②he could not find his ticket. The conductor, noting ③his obvious distress, kindly said, “I know who you are, Dr. Einstein. Don’t worry about your ticket.” Several minutes later the conductor turned around from the front of the traincar to see Einstein continuing to search under ④his seat for the missing ticket. Quickly, he hurried back to assure the gray-haired gentleman. “Dr. Einstein, Dr. Einstein, I know who you are!” ⑤he repeated. “Please don’t worry about your ticket.” Dr. Einstein slowly arose from his knees and addressed the young conductor. “Son, you don’t understand. I, too, know who I am. What I don’t know is where I’m going.”