Homer initially refuses to attend Bart 's show because he discovers the artwork is an insult to him , but changes his mind after Bart apologizes and writes " I 'm sorry " on the hood of Homer 's car . At the show , Chief Wiggum and the Springfield Police Department suddenly appear to arrest Bart for making graffiti throughout the town . It is revealed that the gallery show is a sham and that Fairey is an undercover officer who helped the police identify Bart as the graffiti artist that had been spray @-@ painting Springfield . Since Bart is just a boy , he is not sent to jail . Instead , he is punished by once again being locked up in the rabbit cage . When Bart tells Wiggum that he has to go to the bathroom , Wiggum covers the cage with the blanket and finds Bart gone when he removes the blanket from the cage .