It reminds me of this joke (which I heard my father who's a physician)
There was a chief physician at a renowned medical college who was said to always start the first round of the year for medical students by teaching them on the 'singular imporant principle of medicine'. He would begin by talking about diabetes mellitus: "Diabetes is a Greek name, but the Romans noticed that the bees like the urine of diabetics, so they added the word 'mellitus' which means sweet as honey. As you know, you may find sugar in the urine of a diabetic..."
He then held up a fresh sample of urine just taken from a patient and held up like a trophy, with the students staring at the straw-colored fluid as if they've never seen such a thing before. The physician then suddenly dipped his finger boldly into the urine, and licked his finger with the tip of his tongue. As if tasting wine, he opened and closed his lips rapidly. Could he perhaps detect a faint taste of sugar?
The sample was passed on for the students to try; they all dipped a finger into the fluid, and then licked their finger.
"Now you have learned the first principle of diagnosis, I mean the power of observation" said the physician grinning. "You see," he continued triumphantly, "I dipped my MIDDLE finger into the urine, but licked my INDEX finger, not like all you."