The professor started the class by telling them, "In Veterinary Medicine it is necessary to have two important qualities as a doctor:
The first is that you not be disgusted by anything involving the animal body". The Professor pulled back the sheet, stuck his finger in the butt of the dead cow, withdrew it and stuck his finger in his mouth. "Go ahead and do the same thing," he told his students. The students freaked out, hesitated for several minutes, but eventually took turns sticking a finger in the dead cow's butt & sucking on it.
When everyone finished, the Professor looked at them and said, "The second most important quality is observation. I stuck in my middle finger and sucked on my index finger. Now learn to pay attention. Life's tough, but it's even tougher if you're stupid!"