What secret does Jem share with Scout?
Jem tells Scout that his pants were folded, mended and left for him when he went back to reclaim them, after they were lost in his attempt to escape the Radley property.
When Scout, Jem, and Dill go to look at the Radley house, they're hoping to get a glimpse of Boo Radley. Unfortunately, their presence is detected by Nathan Radley. He thinks the children are a black person and fires into the air to scare them away. They run, but Jem's pants catch on the barbed wire fence. He abandons them.
He goes back to get them that night so that Atticus doesn't catch him in a lie.
For the next week, he doesn't tell Scout anything. She says he's moody and silent. Then he breaks his silence and tells her that when he went back for the pants, they were mended, folded, and laid across the fence. Jem says it was like someone was expecting him to come back for them.