true ancient roman joke

A barber, a bald man and an absent-minded professor are taking a journey together. They have to camp overnight, so decide to take turns watching the luggage. When it's the barber's turn, he gets bored, so amuses himself by shaving the head of the professor. When the professor is woken up for his shift, he feels his head, and says "How stupid is that barber? He's woken up the bald man instead of me."