"Learning about observation" joke

A lecturer teaching medicine was tutoring a class on 'Observation'.
He took out a jar of yellow-colored liquid. "This", he explained, "is
urine. To be a doctor, you have to be observant to colour, smell,
sight and taste."
After saying this, he dipped his finger into the jar and put it into
his mouth. His class watched on in amazement, most, in disgust.
But being the good students that they were, the jar was passed,
and one by one, they dipped one finger into the jar and then put
it into their mouth.
After the last student was done, the lecturer shook his head. "If
anyof you had been observant, you would have noticed that I put
my second finger into the jar and my third finger into my mouth."