One of the world's greatest scientists was also recognized as the original absent-minded professor. 

One day, on board a train, he was unable to find his ticket.  The conductor said, "Take it easy.  You'll find it."

When the conductor returned, the professor still couldn't find the ticket.  The conductor, recognizing the famous scientist, said, "I'm sure you bought a ticket.  Forget about it."

"You're very kind," the professor said, "but I must find it, otherwise I won't know where to get off."