The medical students were attending their first Biochemistry Class. They were gathered around the lab table with a urine sample. The professor dipped his finger in urine and tasted it in his own mouth. Then he asked the students to do the same. The students hesitated for several minutes but at last every one dipped their finger in the urine sample and tasted it. When everyone finished, the Professor looked at them and said "The most important quality is observation. I dipped my MIDDLE finger in the urine but tasted the INDEX finger. Today you just learned HOW TO PAY ATTENTION!"