In the novel To Kill a Mockingbird, Nathan Radley filled the knothole of the tree with cement. What reason did he give Jem for doing this?

In Chapter 7, Jem and Scout receive small gifts in the knothole of the Radley tree. They aren't sure whose been leaving them gifts, but they both decide that it would be a good idea to write the person a thank you note. After writing the letter, Jem goes to leave the note in the knothole of the tree the following morning, only to find out that it has been filled in with cement. Jem is both shocked and upset when he sees the filled knothole. That evening, he waits patiently for Nathan Radley to pass their home, and the next day when Nathan walks by, Jem asks him why he filled the knothole in with cement. Nathan lies to Jem and tells him that the tree is dying. He says, "You plug ’em with cement when they’re sick" (Lee 39). Jem is suspicious about Nathan's reason for filling the knothole and asks Atticus if he thinks the tree is dying. When Atticus says that the tree looks healthy, Jem's suspicions are confirmed, and he realizes that Nathan lied to him. Scout mentions that Jem stood on the porch until nightfall, and when he came inside, she could tell that he had been crying.