What quote reveals Jem's true motivation for creating the Boo Radley game?

In Chapter 4, Scout mentions that Jem believes in Hot Steams in front of Dill which upsets him. After the children agree to roll the tire, Jem gets revenge on Scout by pushing her as hard as he can. Scout ends up crashing into the Radley porch. Scout is dizzy when she exits the tire and suddenly realizes that she is inside the Radley yard. Scout then runs out of the yard without grabbing the tire. Jem works up enough bravery to finally run into the Radley yard and retrieves the tire. When Jem returns with the tire, he comments that there was nothing to it and says that Scout acts so much like a girl that it's "mortifyin'." A little while later, the children are drinking lemonade and Jem tells them that they are going to play a new game called "Boo Radley." Scout says,

"Jem's head at times was transparent: he had thought that up to make me understand he wasn't afraid of Radleys in any shape or form, to contrast his own fearless heroism with my cowardice" (Lee 25).

Jem simply wanted to create the illusion that he wasn't afraid of Boo Radley.