This reminds me of an old trick that consultants used to play on medical students (you'd get sacked now, of course). When a patient has diabetes, the urine is often sugary. The consultant would ask the nurse for a sample of urine in a bowl, dip his finger into it, lick his finger and take a guess at how high the patient's blood sugar was (of course he had looked at the charts earlier). He'd then pass the bowl around all the students how would each dutifully dip a finger in and taste the urine to "learn" how to assess how sugary it is.
Of course, the consultant had dipped one finger in, licked a different finger and hastily washed the urine off.