Found

“He knew it was just a prank—it had to be—but for just a second, staring at those words, You are one of the missing, he’d almost believed them. Especially since he’d just been telling Chip about being adopted…What if somebody really was missing him? He didn’t know anything about his birth parents; all the adoption records had been sealed.”

Jonah thinks this after receiving the first strange note that identifies him as a missing child. He does not know it, but these lines are a call to action. Strange circumstances surround Jonah’s adoption, and he will need to face them. This is the first hint that his past is different from what he has always believed.

“What I meant to say is, that doesn’t matter either. If you’re going through some adolescent ‘Who am I?’ phase, it’s not because you’re adopted. Everyone goes through that. I don’t know who I am either.” 

Katherine says this right before Jonah receives the second strange note. He has spent the last few chapters upset. He feels for Chip, who learned about his own adoption, and now Jonah is wondering where he came from, something he has never given much thought before. This is not the first appearance of the theme “Who am I?”, but Katherine’s admission introduces the idea that all kids feel confused, not only those who were adopted. Katherine questions who she will become, and Jonah wonders who he is. Their dichotomy mirrors the choice Jonah and the other missing kids must make at the story’s end: live in the past or the future.

“He studied so hard that, second period, the test was a breeze. He filled in the meaningless words—Homo erectus, Homo habilis, Homo sapiens, Neanderthal—with great relief. These, at least, were questions he could answer.” 

These are Jonah’s thoughts after receiving the second letter. He has more questions than answers in his life, so knowing the correct responses to the quiz fills him with relief.