In To Kill a Mockingbird, what is "the oldest lawyer's trick on record" that Atticus tells Jem he has used to learn about the boy's game?
Ironically, Chapter 5 of To Kill a Mockingbird opens with Jem's equivocations, a ploy which he again uses unsuccessfully at the end of the chapter with his father after he is caught trying to leave a note for Boo Radley. For, Scout narrates that her nagging at Jem to leave Boo alone has slowed down Jem's plots because Atticus does not approve; however, Jem maintains
...that Atticus hadn't said we couldn't, therefore we could; and if Atticus ever said we couldn't, Jem had thought of a way around it: he would simply change the names of the characters and then we couldn't be accused of playing anything.
Then, after Dill persuades Jem to place a note on Radley's window sill, the boys are suprised by the appearance of Atticus who has returned home to retrieve a file he has left in the house. He scolds his son about "tormenting" Boo Radley telling him also to not play any games or make fun of anybody in their neighborhood. When his father says this, Jem protests,
"We weren't makin' fun of him, we weren't laughin' at him...we were just--"
And Atticus finishes Jem's sentence, "putting his life's history on display for the edification of the neighborhood." For, by denying all else, Jem has so much as admitted to what he has done. By tricking Jem with his scolding, Atticus has caused Jem to concede the only action left, that he was exposing Boo. This is the "oldest lawyer's trick on record."