i heard a variation of this one first with some sort of science class in a lecture room.
the professor had a jar of urine and told them for every process you needed to look at it, smell it, feel it, and taste it so you know everything about it..
and while he said these things, he went through the motions, even so much as taking his finger then sucking on it..
he then told everyone in the class to do the same..
after they were done he said the biggest thing to remember though, is observation... they should have observed he dipped his middle finger in and sucked his ring finger.
still a funny joke.