The mockingbird is an obvious symbol and point of discussion. It is regularly cited for Atticus' comments about its innocence. The mockingbird offers sweet music, but it does not harm anyone. It may be different than the other birds. But, just because mankind has the power to dominate the mockingbird doesn't mean mankind should. The mockingbird is later exemplified in the characters of Boo Radley and Tom Robinson.
The knothole is another important object. This hole represents a tangible relationship between the children and Boo Radley. Mr. Nathan Radley plugs the hole after many gifts are exchanged. This sincerely demonstrates the lengths adults will go to in order to disrupt innocent and positive interaction in the effort to be that person who watches over someone who is supposedly less mature. The children also digest the lie that Mr. Nathan tells about the tree enduring a truth about adult life: lies occur in order to maintain convenience.