Christopher reveals that he thinks he would make an excellent astronaut. He likes computers and machines and outer space and would not be lonely. He would want to take his pet rat Toby with him.
At school he tells Siobhan that he is not allowed to do any more detective work, so his book is finished. She tells him that is fine, but Christopher says it is not complete because it does not have a proper ending. He talks to her about Mr. Shears and Mrs. Shears but she has no answers for him because she does not know them.
Several days later Christopher encounters Mrs. Alexander at a local store. She asks him why he went away the other day, and he replies that he thought she might be calling the police. Mrs. Alexander engages him in conversation and he tells her he is very good at math and will be taking his A-level in it next month.
He also decides to ask her about Mr. Shears. Is he an evil man, as his father has told him? Mrs. Alexander wonders why he is asking her this and says it might be better if he did not, because it might upset his father. She says she thinks he knows why his father does not like Mr. Shears, but Christopher does not. He asks why she said that, but she does not answer at first. Then, as they walk into a park, Mrs. Alexander explains that his mother and Mr. Shears were “very good friends.” Christopher asks if they were “doing sex,” although he does not seem to have any understanding of what that implies. Mrs. Alexander confirms that they were, and that is why his father thinks Mr. Shears is evil. Christopher deduces that that was why Mr. Shears left his wife. Mrs. Alexander makes him promise not to tell his father about their conversation.
Christopher shows his interest in and talent for mathematics and logic by telling a story about what is known as the Monty Hall Problem. The problem came up in a magazine column written by a woman named Marilyn vos Savant, who reportedly had the highest IQ in the world. The question was sent in by a reader:
You are on a game show on television. On this game show the idea is to win a car as a prize. The game show host shows you three doors. He says that there is a car behind one of the doors and there are goats behind the other two doors. He asks you to pick a door. You pick a door but the door is not opened. Then the game show host opens one of the doors you didn't pick to show a goat (because he knows what is behind the doors). Then he says that you have one final chance to change your mind before the doors are opened and you get a car or a goat. So he asks you if you want to change your mind and pick the other unopened door instead. What should you do?
Marilyn responded that you should change and pick the final door, because there is a two in three chance of getting the car. Many people wrote in saying she was wrong, but Christopher explains in great detail why she was correct. He concludes that logic is always better than intuition, because intuition would tell you that without changing, your chances of winning would be 50-50.
Analysis, Chapters 83-101
Chapter 83 is another of the digressions in which Christopher gives out information about himself and how his mind works, in this case, the fact that he thinks he would make a good astronaut. The chapter does not advance the plot but helps to create a more detailed picture of who Christopher is (he likes being alone and he loves machines and computers). Chapter 101 serves a similar purpose. The Monty Hall Problem is not fiction; it refers to a real controversy that took place exactly as Christopher describes. Christopher use of it here emphasizes yet again, if that is necessary, that this fifteen-year-old boy has a grasp of mathematics, and an ability to solve problems by using it, that would be considered quite extraordinary for a boy of his age.