Final answer:
Atticus discovers the children's game about Boo Radley when he notices them with scissors, acting out the Radley family's life. He reprimands them, emphasizing the importance of respecting the Radleys' privacy.
Step-by-step explanation:
Atticus finds out with certainty that the children were playing a game about Boo Radley when he catches them with scissors, enacting a play that evidently mirrors the rumored events of the Radleys' lives. He realizes that the scissors are not for their intended purpose but are being used as props in their game. Atticus firmly speaks to the children about respecting the Radleys' right to privacy, highlighting that what the Radley family does is their own business and not for the children to turn into entertainment or gossip.