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's body."
The students freaked out, hesitated for several minutes, but eventually took turns sticking a finger in the butt of the dead cow and 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."