Got You

A lecturer teaching medicine was giving a classoom 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."