Playpen balls are found in a Ball pit and are a symbol of childhood. Many people grew up playing in playpens filled with these balls. Unfortunately, many people believe that in order to "grow up" they have to cast off all of these fun items. Randall takes a different stance, growing up is merely being able to appreciate all the fun things better.
China currently holds the record for the worlds largest ballpit: a swimming pool filled with 1 million balls.
A friend of Randall's has made a web app for calculating how many balls it would take to fill your apartment.