Christopher decides to try to solve the mystery of Wellington’s death even though his father has told him not to. He explains that he doesn’t always do what other people tell him to, because people give confusingly vague instructions and because people constantly break rules, like driving over the speed limit. Ed told him to “stay out of other people’s business,” but people’s business could mean anything. Siobhan understands this problem, so she always gives him very specific instructions concerning what he should and shouldn’t do.
Christopher again points out the absurdity of what most people see as normal human interactions. People who are not autistic take these interactions and unspoken rules for granted in a way that Christopher cannot. As a result, Christopher makes up his own rules. Siobhan is beginning to emerge as the character who best understands the way that Christopher’s mind works.
That evening, Christopher knocks on Mrs. Shears’ door. She tells him she doesn’t want to see him, but he replies that he didn’t kill Wellington, and he wants to find out who did. When he asks if she knows who killed the dog, she doesn’t answer, and only closes the door in his face.
At this point, it seems that Mrs. Shears might actually blame Christopher for her dog’s death. Later, it will become clear that she has other reasons to be upset, and she in fact probably does know who killed Wellington—as she gives no answer here.
Christopher sneaks around to Mrs. Shears’ garden shed. It’s locked, but through the window he can see a pitchfork that looks like the one that killed Wellington. He wonders if Mrs. Shears killed her own dog, but decides that the murderer was probably someone else using her pitchfork. However, the shed is locked, so the killer might have had the key. Mrs. Shears appears and tells Christopher she’ll call the police if he doesn’t leave immediately. He goes home, contented with his detective work.
Christopher acts like a real detective in a story, snooping around to find the murder weapon. He considers possibilities with a cool mind, not ruling Mrs. Shears out as a killer simply because Wellington was her dog. Furthermore, he again demonstrates his unconcern for other people’s emotions; he’s not worried when Mrs. Shears threatens to call the police, instead feeling satisfied with himself.