The professor loved to start each lecture with a dirty joke. After a week of this, the female students got together and decided that next time, if this happened again, they would all walk out in unison.
The professor got wind of this plot. Next morning, after he entered the lecture hall, he said: "Good morning! Have you heard about the shortage of prostitutes in Singapore?"
Now all the female students stood up and headed toward the exit.
The professor continued: "Oh, ladies, please wait, the boat to Singapore doesn't leave until tomorrow!"
