One day, the teacher walks into her classroom and announces to the class that on each Friday, she will ask a question to the class and anyone who answers correctly doesn't have to go to school the following Monday.

On the first Friday, the teacher asks, "How many grains of sand are in the beach?" Needless to say, no one could answer.

The following Friday, the teacher asks the class, "How many stars are in the sky?" and again no one could answer.
Frustrated, little Johnny decides that the next Friday, he would somehow answer the question and get a 3 day weekend.

So Thursday night, Johnny takes two ping-pong balls and paints them black. The next day, he brings them to school in a paper bag.
At the end of the day, just when the teacher says, "Here's this week's question,"
Johnny empties the bag to the floor sending the ping-pong balls rolling to the front of the room.
Because they are young kids who find any disruption of class amusing, the entire class starts laughing.

The teacher says, " Okay, who's the comedian with the black balls?"

Immediately, little Johnny stands up and says, "Bill Cosby, see ya on Tuesday.