At the end of chapter 5 there is talk about the oldest lawyer trick on record. What is it?
Towards the end of chapter 5, Atticus returns home from work unexpectedly and catches Jem attempting to place a letter attached to the end of a fishing pole in the Radley's window sill. Atticus proceeds to chastise Jem and the children for continually bothering Boo Radley. After Atticus questions Jem's decision to intrude on Boo's privacy, he chastises the children for putting Boo's life on display by playing the "asinine game" in the front yard for everyone to see. Earlier, Atticus had caught Jem and the children playing the Boo Radley game in the front yard, and Jem denied that their game had anything to do with Boo. Atticus uses the "oldest lawyer’s trick on record" by assuming Jem was playing a game involving Boo and rapidly questioning his son, which essentially tricks Jem into revealing the truth about their game. Jem responds by telling Atticus,
We weren’t makin' fun of him, we weren’t laughin’ at him . . . we were just (Lee, 50).
Atticus essentially catches Jem in a lie by asking him a series of leading questions in quick succession, which is the oldest lawyer trick on record. Jem is caught off guard and admits to something he denied earlier.
Atticus' trick is to get Jem to admit things (specifically, that he and the other kids had been playacting Boo's life for fun) by asking a series of leading questions until Jem gets knocked off balance (mentally) and accidentally admits to doing what Atticus already knew they'd done.
The trick, then, is to confuse the person through leading questions until he or she admits things.