There was a professor who would start off his class with a dirty joke. Some of the girls in his class were fed up at this and decided to leave as soon as he started to tell his joke the next class.
The professor caught wind of their plan and the next class he began his joke and said, "Lately there's been a critical shortage of whores in China." The girls all stood up and began to walk towards the door. And the professor said, "Whoa hold on girls, the boat doesn't leave till tommorow!"