The climax in this brilliant book is when Natalie meets 105 just before her appointment with Esther P. Marinoff. This is the decisive moment in the story after which the chain of events is set and determined. After witnessing this, Moses cannot keep silent.
This then leads to the falling action during which Moose forces his parents to see that they must stop trying to deceive everybody about Natalie's age, and that the only chance she will have at her special school is if they are honest and they tell them about how old Natalie really is. When Mr Flannagan asks Natalie her age, her precise response, "I am sixteen at two thirty-one today," clearly reveals not just her age but also her literal way of dealing with facts. Note how her father responds:
My father presses his lips so hard together, they turn white. The tears are falling again, so fast, it looks as if he can't see.
The emotional intensity of this moment is conveyed through the body language of the father, who is not only crying, but who has pressed his lips together so hard that "they turn white." This is a moment of emotional release both for him and the whole family, as they commit themselves to telling the truth and being honest, which could not have occurred without the decisive moment between Natalie and 105, the climax, witnessed by Moses.