"little Johnny" joke

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 on 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 you on Tuesday!"