An old one ... but a still a good one to read...

First-year students at Med School were receiving their
first anatomy class with a real dead human body.
They all gathered around the surgery table with the body
covered with a white sheet.
The professor began the lecture by telling them:
"In medicine, it is necessary to possess two important
qualities as a doctor: The first is that you not be
disgusted by anything involving the human body."
To illustrate, he pulled back the sheet, stuck his finger
in the anus of the corpse, withdrew it, and stuck it in his
mouth."
(YUCKS)
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 butt of the
dead body 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"