In the classic novel, Jem and Scout make it their mission to try to get Boo Radley to come out of his house. They had heard all the rumors about him, and they wanted to see him for themselves. Atticus had told them many times to leave the Radley's alone, but they are kids and are very curious.
Jem and Scout find several things in the knot-hole of the old oak tree on the Radley property. The children are very curious as to who has left these things there. Later in the book they realize that it is Boo who is leaving these little gifts. He is trying to show them his affection for them. In the knot-hole they find 2 sticks of gum, a pack of gum, 2 old Indian head coins, gray twine, soap figures carved to like Jem and Scout, old spelling bee medals and an old watch on a chain and an aluminum knife.
"Two live oaks stood at the end of the Radley lot; their roots reached into the side road and made it bumpy. Something about one of the trees attracted my attention. Some tin foil was sticking out of a knot-hole just above my eye level, winking at me in the afternoon sun. I stood on my tiptoe, hastily looked around once more, reached into the hole, and withdrew two pieces of chewing gum minus their outer wrappers."
Jem and Scout don't realize it yet, but the person leaving these things for them will be the person who saves their lives. The person they have spent so much time trying to get to come out of his house, is the person who watches them and feels a deep connection to the both of them.