A professor started his medical class by telling his freshman students "In the medical field it is very important to have two qualities as a doctor. The first is that you not be disgusted by anything involving the human body." For example, the professor pulled back the sheet revealing a dead body, he stuck his finger in the corpse's @ss, pulled it out and stuck it in his mouth to suck on it. "Now you do the same", he told his student.
The students freaked out, hesitated for a few minutes but eventually each took turn doing it... followed by gagging, retching and spitting, etc.
When everyone is finished wiping their faces and mouths, the professor looked at them and said, "The second most important quality is observation. I used my middle finger to stick it in but sucked on my index finger. Now learn to pay attention."