A lecturer teaching medicine was tutoring 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 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."