Lesson Learned

 A lecturer teaching medicine was giving a class on observation. He
took out a jar of yellow liquid.

"This," he explained, "is urine. To be a doctor, you have to be
observant of color, smell, sight, and taste."

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

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 second
finger into the jar and my third finger into my mouth."