In chapter 31, Scout reminisces about what she and her brother found in the oak tree from Boo Radley. They first find gum, which Jem makes her spit out because he thinks it is poisoned. Next, they find the following: two Indianhead pennies, two soap dolls, and a broken watch with a chain and a small knife. By the end of the book, though, Scout knows that Boo Radley is a friend and neighbor, not the boogeyman that she was first taught about by Jem and Miss Stephanie Crawford. Consequently, Scout says the following about Boo Radley:
"Neighbors bring food with death and flowers with sickness and little things in between. Boo was our neighbor. He gave us two soap dolls, a broken watch and chain, a pair of good-luck pennies, and our lives. But neighbors give in return. We never put back into the tree what we took out of it: we had given him nothing, and it made me sad" (278).
This passage shows that Scout is very grateful for the material items that Boo Radley gave them through the knothole of the tree. (In fact, in chapter 7, she and Jem write a thank you letter to put into the knothole of the tree, but Mr. Nathan Radley filled it up with cement before they could accomplish their goal.) She also appreciates the fact that she and Jem were saved by Boo on the night that Bob Ewell attacked them. However, Scout also feels sad because she is not able to give anything to him in return. Therefore, once Boo saves the children's lives in chapter 28, there doesn't seem to be anything more they could give him in response to all of his kindnesses, so this makes her sad.