Atticus used the "the oldest lawyer’s trick on
record" to trick Jem into admitting he played the Radley game.

Atticus uses a lawyer's cross-examination trick on Jem. First, he asks Jem innocently enough why he wants Boo Radley to come out. From there, he manipulates the conversation so Jem eventually admits to playing the Radley game. 

Atticus tells Scout and Jem to leave the Radleys alone. The Radleys, Atticus says, are not peculiar and should be left alone. They deserve privacy.

Lastly, we were to stay away from that house until we were invited there, we were not to play an asinine game he had seen us playing or make fun of anybody on this street or in this town—

“We weren’t makin‘ fun of him, we weren’t laughin’ at him,” said Jem, “we were just—” (Chapter 5) 

At this point, Atticus jumps in and twists Jem’s words, pointing out that he admitted he was playing the Boo Radley game when he protested that he was not making fun of Boo Radley. If he wasn’t playing the game, then who was he not making fun of? This is why Atticus is a good lawyer. 

“So that was what you were doing, wasn’t it?”

“Makin‘ fun of him?”

“No,” said Atticus, “putting his life’s history on display for the edification of the neighborhood.”

Jem seemed to swell a little. “I didn’t say we were doin‘ that, I didn’t say it!”

Atticus grinned dryly. “You just told me,” he said. “You stop this nonsense right now, every one of you” (Chapter 5).

Jem is upset at Atticus for tricking him, but Atticus reminds him that he said he wanted to be a lawyer. Jem calls after Atticus that he is not so sure. He does not appreciate being manipulated by his father. It must not be easy to have a dad who is a lawyer after all!