A teacher and his students in the Anatomy class.

A teacher tells his students at their first day of the Medicine course, during the Anatomy class:

"The first thing one has to do to become a good doctor is to be a good observant. The second one is to never, ever feel disgusted by anything."

After this observation, the teacher leads the students to the autopsy room, where there is a corpse on a table.

"Let's do our first test. Everybody must do exactly what I'll do. Did I make myself clear?"

That being said, the teacher sticks his finger in the corpse's butthole, licking it soon after. The students are disgusted by the scene but follow the teacher's instructions. Each of them sticks their fingers in the corpse's butthole and then lick their fingers. After all of them have done it, the teacher tells his students:

"I want to congratulate you on not feeling disgusted by anything. That's essential to become a good doctor. However, you need to improve your observation skills. I stuck one finger in the butthole and licked another one."