Little Johnny is in a class where every Friday the teacher asks a question, and if you get it right you don't have to go to school on Monday.
The fist friday the question was how many gallons of water are there in the whole world. No one knew so they all had to go to school on Monday.
Next Friday the question was how many grains of sand are there in the whole world. No one knew so they had to sgo to school on Monday.
By this time Little Johnny is getting mad because he doesn't want to go to school on Monday, so he paints two ping-pong balls black and the next Friday right before the teacher asked the question he rolled the two black ping-pong balls up to her and she said, "Who is the comedian with two black balls?"
Little Johnny said," Bill Cosby. See you on Tuesday."