Power of Observation

A professor teaching medicine is tutoring a class on 'Observation.' He then takes out a jar of yellow-colored liquid. "This," he explains holding up the jar, "is urine. To be a good doctor, you have to use all your 5 senses.” After saying this, the professor dips his finger into the jar and puts it into his mouth.
His class watches on, more in disgust than in amazement. But being the diligent students that they are, as the jar is passed from one student to the other, one by one, they dip one finger into the jar and then put it into their mouth.
After the last student is done, the professor shakes his head and says, "If any of you had been observant, you would have noticed that I put my 2nd finger into the jar and my 3rd finger into my mouth