What dare of Dill's does Jem finally accept?
Dill is fascinated with Boo Radley. Scout and Jem are fascinated as well but they know all of the gossip and false stories about Boo being some kind of monster, so they are terrified of getting anywhere near the Radley house. Even though Jem describes Boo as a monster with bloodstained hands and yellow teeth, Dill still wants to get a glimpse of Boo. In order to make this happen, Dill bets Jem one book against Jem's two that Jem will not go past the Radley's gate:
Our first raid came to pass only because Dill bet Jem The Gray Ghost against two Tom Swifts that Jem wouldn't get any farther than the Radley gate. In all his life, Jem had never declined a dare.
Dill pesters him for three days until Jem finally accepts the dare. To make it a little more enticing, Dill alters the bet. "I won't say you ran out on a dare an' I'll swap you The Gray Ghost if you just go up and touch the house." Finally, Jem runs through the gate, slaps his palm on the house and runs back to his porch. The only reaction they get is a small movement of the shutter in a window of the Radley house. But it is enough to keep them fascinated with Boo Radley.