An autopsy professor was giving an introductory lecture to a class of students. Standing over a corpse, he addressed the class. 'There are two things you need to make a career in medical forensics. First, you must have no fear.' Having said that, he shoved his finger up the corpse's anus and licked it. 'Now you must do the same,' he told the class.

After a couple of minutes of uneasy silence, the class did as instructed.

'Second,' the professor continued, 'you must have an acute sense of observation. For instance, how many of you noticed that I put my middle finger up this man's anus, but licked my index finger?'