An eminent surgeon is standing over a corpse and says to his gathered medical students "There are two attributes to becoming good doctors. One is respect for people who are ill and the second is good observation."
At this point he sticks a finger up the corpe's bottom and then puts his hand his mouth and licks a finger.
"There he says. Now you do it."
The students one by one, although astonished place a digit in the appropriate place and then licks that finger.
When the last has finished the proffessor says to the gathered crowd, "You see one must have respect and one must be observant. Those of you who are observant - and none are- will have noticed I used my index finger to probe and my middle finger to lick!"
"Let that be a lesson to you."
anon.