 Lisa has a nitrous oxide @-@ induced hallucination that echoes The Beatles film Yellow Submarine , which Al Jean says had to be changed slightly for legal reasons . This included changing the name of the short sequence to " purple submersible " ; George Harrison calls her " Lisa in the Sky " , while Paul McCartney notes " no diamonds though " . The scene where Lisa acquires her monstrous braces and breaks her mirror is based on a scene from the 1989 Tim Burton film Batman where Jack Napier discovers his transformation into the Joker . 
