Here's another powerful image, as the rat's physical isolation in the cage parallels Christopher's emotional isolation. We might even compare Toby's scratching to Christopher's uneasy processing of his realization about his mother being alive. Deep.

Christopher has a difficult time determining when people are joking, which makes it especially interesting to imagine him sort of putting his fear on stand-by until he decides whether or not his dad is joking (about killing Wellington the dog). When he realizes his pop's not joking, he makes an immediate (logical, in a way) connection between his dad killing a dog and the thought that his dad would be capable of hurting him as well.

Christopher runs away from home, but he only gets to the backyard before feeling a little overwhelmed by his newfound freedom. What does he do, then? Well, he crams himself into a nice tight squeeze behind the garden shed, and, safely confined, feels much more secure.