There was this medical school, where students studied to become doctors. One day, their professor brought them to the lab for a session, where there was a dead corpse inside. The professor said, "There are two things you need to succeed as a doctor. One, always follow the professor's instructions. Watch me." The professor placed his index finger into the dead corpse's anus, and placed it in his mouth. "Do what I did. You may begin.", the professor said to his students. The students were shocked at this disgusting thing. But since they really wanted to become a doctor, they just did it. One by one, they repeated what the professor did. When everyone had done it, the professor said, "The second thing you need to have to succeed as a doctor is to have a keen eye for observation, which all of you lack. Did you notice, that I placed the index finger into the corpse's anus, but I placed my thumb into my mouth?"