In Chapter 7 of To Kill a Mockingbird, Scout and Jem discover in the knot hole of the Radley tree two carved figures that resemble them; two weeks later, they find chewing gum.

When they first discover that items are in the knot hole of the tree, Scout acts with superstition: as she pulls two small soap figures out from the knot hole, she screams and throws them down on the ground. Angrily, Jem scolds her and picks up the figures. "These are good," he says, and then Jem and Scout realize that the figures resemble themselves. 

Shortly thereafter, there are other gifts in the knot hole, such as a pack of chewing gum. So Jem and Scout decide to write a letter thanking the donor for his gifts. After they compose their thank-you letter, the children place it into the hole.

Next morning on the way to school, he ran ahead of me and stopped at the tree. Jem was facing me when he looked up and I saw him go stark white.

Scout runs to Jem and discovers, too, that the hole has been filled with cement. Later, Jem talks with Mr. Nathan Radley, who admits to having filled it. He says that the tree was dying, so he "plugged it" with cement. It is then that readers realize the oppression under which Boo lives.

Scout finds sticks of gum in the knot hole of a tree on the Radley Place in Chapter Four: "Two live oaks stood at the edge of the Radley lot...Some tinfoil was sticking in a knot-hole just above my eve level, winking at me in the afternoon sun. I stood on tiptoe, hastily looked around once more, reached into the hole, and withdrew two pieces of chewing gum minus the outer wrappers."

Boo has done this, of course, as a gesture of kindness and to get the children to come more frequently to the property.