"Fat Man and Little Boy" is the fifth episode of The Simpsons' sixteenth season. It aired on December 12, 2004 and the last new episode of 2004. It was written by Joel H. Cohen and directed by Mike B. Anderson.
Bart refuses to accept that he is no longer a boy after he loses his last baby tooth, so he decides to express himself with T-shirt slogans, which bring him to the attention of a local novelty salesmen interested in becoming Bart's partner, causing the recently-fired Homer to consider himself the number two man of the house; a documentary on lions leads Homer to believe that the only way back to the top is to re-establish himself with another young cub, so he turns Lisa's science project - a model of the first nuclear reactor - into a working model, complete with plutonium he "acquired" from the plant. 
Bart fires spitballs at Lisa and Janey and one is covered in blood. Lisa then complains to Bart about it, but Bart finds out he is losing his last baby tooth. Bart then starts to worry that he is becoming an adult and has to give up his life as a kid. Lisa then tells Bart that when she is feeling down, she writes stuff, so Bart decides to write amusing phrases on T-shirts. They very quickly become very popular and Bart opens his own T-shirt stand, but Chief Wiggum shuts it down, so Bart goes to the novelty expo to increase sales. There Bart finds out that Krusty is selling better T-shirts than him, and Bart gives up. However, a businessman named Goose Gladwell finds Bart's T-shirts and distributes them. Homer then realizes he just lost his spot in the family and ends up watching a Declan Desmond movie on TV. The movie then tells Homer instead of moving to the Springfield Retirement Castle, he can spend time with Lisa. Homer enjoys spending time with Lisa. But after Lisa's project on nuclear energy is about to lose to Martin's robot named CHUM (short for "Childlike Humanoid Urban Muchacho") and Homer uses real plutonium to power Lisa's project, Lisa begins to hate Homer again. In the meantime, Goose Gladwell is selling the rights to Bart's T-shirts to the Disney company, and Bart is not even given a single penny. Homer then comes to the store and after a discussion with Goose, Homer is able to end sales to Bart's T-shirts and Homer is able to reprise his role as father again. During the credits, Bart tells Nelson that there is more than one way to remove a tooth.