S2·E12 The Boy Who Would Be Queen

Timmy tells Trixie Tang that he has a great gift for her, so he can be invited to her party. Obviously, he's lying. Later that day, Timmy and Cosmo start making of girls, and Wanda becomes so angry she turns Timmy into a girl. Timmy decides being a girl can help him/her figure out what Trixie would like for her birthday, so he/she goes off to the mall...but not without revenge on his godparents for turning him into a girl...he wishes Cosmo was a woman and Wanda was a man.