I liked the fact that Rick Springfield (who I hadn't heard of before this series!) lived in another universe beyond the blackboard of the classroom. Rick used the gramaphone to communicate with the class before the portal was opened. I remember two episodes: one where Rick was in a "backwards" universe, and he was shouting "Cylinder the Reverse!" until the class figured out how to understand what he was saying. Another involved some people who stole the door from Rick's side of the doorway, but the door in the classroom was still there. The children had to replace the other door before someone closed the doorway, or there would be no way for them to get back. Interesting take on inter-dimensional travel!