What dare of Dill's does Jem finally accept in To Kill A Mockingbird?

The answer to this question appears in Chapter One of To Kill a Mockingbird. When Jem and Scout meet Dill, who is from out of town, they tell him the entire history of the Radley house, including that of Boo, who all the neighborhood children, of course, are terrified of. So Dill dares Jem to run up to the house and knock on the front door. After a couple days of goading by Dill, Jem finally agrees to a modified version of the dare: he will simply sneak up and touch the house. He finally brings himself to do it, and the children sprint away from the house, where they notice, Scout says, a shutter moving. This is the first introduction to the children's fateful (although until the end, very distant) relationship with Boo, who by the end emerges as one of the most remarkable characters in the book.