The power of observation

A famous Viennese surgeon told his students that a surgeon needed two gifts: freedom from nausea and the power of observation.

Then he dipped a finger into some nauseating fluid and licked it, requesting each of the students to do the same. They steeled themselves to it and managed it without flinching.

With a smile, the surgeon then said, "Gentlemen, I congratulate you on having passed the first test. But not, unfortunately, the second, for not one of you noticed that the finger I licked was not the one I dipped into the fluid."

Anthony de Mello in "The Prayer of the frog" page55