Observation is the key

A lecturer is tutoring a medical class on “observation”. He takes out a jar of yellow liquid. “This,” he says, “is urine. To be a doctor, you have to be observant of colour, smell, sight, and taste.”

He dips his finger in and sticks his finger in his mouth. His class watches in disgust. But being good students, one by one they dip a finger into the jar and put it in their mouth.

After the last student has done it, the lecturer shakes his head.
“If any of you had been observant,” he says, “you would have noticed that I put my second finger into the jar and my third finger into my mouth.”