Nicked by a Name

Nicked by a Name is the first half of the 3rd episode of Season 14. It first aired on October 13, 2010 with The Play's the Thing 

Summary
While playing a good game of soccer, Brain compliments Francine's brilliant play by giving her the nickname "Francine the Soccer Queen." All the other kids want nicknames after that and Brain gets tired quickly of trying to think up suitable names. Arthur keeps asking Brain for his nickname, but Brain never has the time. He gives him the name "Average Arthur" and everybody starts to call him that. At the end of the show him and his friends have a soccer name against Mighty Mountain, who call them by their nicknames. Brain decides to make a sudden choice; give them cool soccer nicknames.

Goofs
In the opening scene Muffy is shown cheering the soccer team on wearing a skirt, but by the end of practice she is depicted wearing normal soccer shorts like everone else on the team!