In chapter 4, what game did the children make up?

They invent a new game about Boo Radley. Jem plays Boo, Dill plays Mr. Radley, and Scout plays Mrs. Radley. They polish it up over the summer into a little dramatic reenactment of all the gossip they've heard about Boo and his family, including a scene using Calpurnia's scissors as a prop. One day Atticus catches them playing the game and asks them if it has anything to do with the Radley family. They deny it, and Atticus replies, "I hope it doesn't." Atticus's sternness forces them to stop playing, and Scout is relieved because she's worried for another reason: she thought she heard the sound of someone laughing inside the Radley house when her tire rolled into their yard.