what was the lawyers trick Atticus used on Jem

Atticus arrives and catches them all. He tells them to stop tormenting Boo, and lectures them about how Boo has a right to his privacy, and that they shouldn't go near the house unless they're invited. He accuses them of putting Boo's life history on display for the edification of the neighborhood. Jem says that he didn't say they were doing that, and thus inadvertently admits that they were doing just that. Atticus caught him with "the oldest lawyer's trick on record." In this, Atticus played on Jem's sympathy and conscience...... inadvertantly he confesses.