In Chapter 4, things turn awry when Jem pushes Scout too hard as she rolls along in an old tire. Scout ends up being dumped out onto the steps at the Radley Place after she comes to a stop.
Meanwhile, Jem yells at his sister to get up and to run away. However, Scout is still dazed due to her stint in the rolling tire, and she struggles to regain her equilibrium. When she manages to become conscious of her surroundings, Scout takes off running. In all her efforts to get away, she leaves the old tire behind, and Jem, to his consternation, has to retrieve it. Later, at the end of the chapter, Scout reveals why she doesn't want to continue playing games which incorporate impersonating the Radleys in some form: she had heard laughter coming from the Radley house on the day she fell out onto the steps at the Radley place. To Scout, Boo Radley is more real than Jem realizes.