Dirty old professor joke

There was an old professor who has a habit of starting his class with a vulgar joke. After one particularly bad example, the women in the class got together and decided to walk out the next time he started. The old professor heard about this plan, and so the next morning he walked in and said, "Good morning, class. Did you hear the one about the shortage of prostitutes in China?" With that, all the women in the class stood up and headed for the door. "Wait, ladies," cried the professor. "The boat does't leave until tomorrow."