A lecturer teaching medicine was tutoring a class on 'Observation'. He took out a jar of yellow-coloured liquid. He explained, "This is urine. To be a doctor, you have to be observant to colour, 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 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 2nd finger into the jar and my 3rd finger into my mouth."