In "To Kill a Mockingbird" why does Jem spill all his secrets to Atticus at Atticus' suggestion that the blanket be returned?
When Atticus has Jem and Scout come watch the fire, he gives them strict instructions to stay right there on the sidewalk next to the Radley house, and not to move; he was probably worried they'd get in the way or get hurt. So, they stay put. But mysteriously, a blanket appears on Scout's shoulders. Atticus questions them about it, and they have no idea how it got there. When it is mentioned that Mr. Nathan Radley might have put it there, Atticus tells Jem to go get some wrapping paper so that they could return the blanket. It is at this point that Jem goes a bit crazy, babbling all sorts of stuff about their excursions on the Radley property. Scout states:
"Jem seemed to have lost his mind. He began pouring out our secrets right and left in total disregard for my safety if not for his own, omitting nothing, knot-hole, pants and all."
He is probably afraid that if they do return the blanket, Nathan Radley will tell Atticus what the kids have been up to in his yard, and they'll get in trouble. So, he confessed first, probably figuring that they'd get in less trouble that way. Secondly, he is probably a bit afraid of encountering Boo in the house somehow; going over there might increase the odds that they see him. And maybe if they do, Boo will spill the beans on all of the mischief that they have caused. So, Jem 'fesses up, laying it all out there before Atticus has to hear it from the Radley men. He's so incoherent and babbling though that Atticus doesn't really grasp his meaning (or chooses to ignore it), and just says they can return the blanket the next time they see Boo.