I had a similar event in High School more years ago than I care to admit. I never forgot it. My Science teacher held a vial labelled “Urea Salt” and he said it had been distilled from human urine. He proceeded to wet his finger, dip it in the vial, then lick his finger. He then challenged anyone else to do it. A lad of my slight acquaintance [not the sharpest pencil in the box!] took up the gauntlet and dipped in his wetted finger. Before he could lick it, the teacher grabbed his hand and explained that he’d wetted and dipped his middle finger but licked his index finger.