Picture this scene: A bunch of first year medical students are in lab having their first experience with a cadaver (that is, a human body donated for educational purposes). Lab has just started, and the professor has just begun to lecture.
"Two things are needed to succeed as a doctor. The first is to not be grossed out by anything involving the human body."
The professor then, to the amazement of his students, sticks his finger into the cadaver's rectum, pulls it out, and begins to suck on it. He then turns to his students and says, "Ok, your turn".
Naturally, the young students are a little freaked out by this challenge from their professor, but after some protest, eventually all of them do as they are asked. Once they had all finished, the professor continues his lecture:
"To reiterate, the first thing you need to become a good doctor is to not be grossed out by anything involving the human body".
The students nod in agreement. After a pause, the professor holds out his hand.
"Now", he says, "The second thing you need to become a good doctor is good observation skills. You see, I stuck my index finger into the cadaver's rectum and sucked on my middle finger. Now, learn to pay attention!"