Episode: 500 Keys (S22, E21) After discovering a set of keys which lead to every door in Springfield, Lisa in typical Lisa fashion explores a hidden classroom. This classroom is jam-packed not with just computers and a science centre but also a rather awesome reference to Avatar which is bound to please any fans of The Last Airbender. However, the joy of Avatar fans was quickly replaced by frustration as the show had muddled the order of the cycle; switching around air and fire. Quickly the taste of a delicious, covert Easter Egg turned to ash in the mouths of the fans of the anime-style cartoon show.