OBSERVATION IS THE KEY...

A medical professor was teaching a class on
'Observation.' He took out a jar of yellow-colored
liquid. "This," he explained, "is urine. To be a
doctor, you have to be observant to color, smell,
sight, and taste."

After saying this, he dipped his finger into the jar
and put it into his mouth. His class watched on in
amazement, most, in disgust. But being the good
medical students that they were, the jar was passed,
and one by one, they dipped one finger into the jar
and then put it into their mouth.

After the last student was done, the lecturer shook
his head. "If any of you had been observant, you would
have noticed that I put my index finger into the jar and my
middle finger into my mouth. Have a good lunch. Class
dismissed!"