In chapter 16 of To Kill a Mockingbird, what does Atticus tell Jem and Scout not to do? And what do they do?

Atticus orders Jem and Scout to stay out of town on the first day of the Tom Robinson trial in Harper Lee's To Kill a Mockingbird. And the children obey his directions all morning, watching the crowds go by (and gossiping about most of the peope) before joining Atticus for lunch at home after the selection of the jury. However, with their curiosity peeked, they can't resist the urge to wander to the courthouse to peek inside. After securing a seat in the Negro section upstairs, they witness exactly what Atticus did not want them to see. They see Bob Ewell in all his trashy glory. They hear first-hand the testimony of his daughter Mayella, and then they see Tom Robinson tell his side of the story. It was all too much for Dill, who begins to cry, so Scout leads him outside to cool off under "the fattest live oak and we sat under it." There they learn the secret that Dolphus Raymond has kept from the rest of the town.

It is precisely what Atticus had hoped to keep from his kids: Tales of sexual transgressions and violence; "alcoholic" outcasts and, later, the true explanation from Tom that the jury refuses to accept. They are the only children in attendance, and Atticus knows beforehand what will occur. For Jem and Scout and Dill, it is yet another lesson of the cruelties of the adult world, one in which all three children come to witness so very early in their lives.