filled world  of TV to which American children are exposed, with a main plot line  being interrupted by frequent commercials hawking educational  concepts instead of products and simulated TV programs. The show  also made extensive use of the reruns concept by replaying popular  segments over and over, intermixed with new material. As a result,  children viewing the show in 2002 will still see the occasional  segment that was originally created for the series when their  parents were still children! Many songs written for the series are  now considered standards. These include "Sing," "Being Green,"  "Rubber Duckie," and "C is for Cookie," as well as the shows theme  song. However, when the show changed formats in 2000, this concept  is less frequently used than before.