A medical lecturer was teaching a class on 'Observation'.
He produced a jar of yellow liquid. "This is urine," he explained. "To be a doctor, you must be observant of color, smell, sight, and taste."
After speaking, he dipped a finger into the jar and then put it into his mouth.
The class watched in amazement, most with disgust. Yet, being diligent students, they passed the jar. One by one, each dipped a finger into the liquid and then into their mouth.
When the last student finished, the lecturer shook his head. "If any of you had been truly observant," he said, "you would have noticed that I dipped my second finger into the jar, but put my third finger into my mouth."