Half related: reminds me of my physics teacher's test of how observant we were. The extra credit question on the test was "what is your teacher's favorite color?", which she had so far given no indication of. But while watching us she was walking all over the room in every possible direction, because the answer was on a piece of paper taped to her back.
My dad had a class in medical school where the professor dipped his finger in a beaker, talked about mechanisms for sugar in diabetic patients’ urine, and then proceeded to stick his finger in his mouth. He had all of the students do the same, who noted that the sugar was apparent.
He concluded the class by talking about the importance of observing patients, and pointed out that he had tasted a different finger than the one he had put in the beaker.
