On the wall across from her bed, a whole galaxy of glow-in-the-dark stars comes to life, and my heart speeds up. It’s the one and only thing our rooms have in common, except mine are on the ceiling. I think of all the times I wished I could step into those stars — fly through them into an alternate universe that’s easier to live in. Maybe Zoe dreams of that, too.

“What do you want to do?” I ask, glancing up at her. “In life, I mean. Do you know?”

She shakes her head. “Not really. Do you?”

“Yeah. I want to be an aerospace engineer,” I say, feeling stupid about how nerdy it sounds.

“Wow, like spaceships and stuff?” she asks.

“Maybe. But I’m also interested in rovers like the ones they’ve put on Mars — mostly how we get them there, how we make sure they can navigate and land safely, that kind of stuff,” I mumble.

“Wow. I have no idea what’s even involved in that.”

I settle back into the chair. “Can I ask you something?” I say, before I can stop myself.

“Sure.”

“Why can’t you swear?”

She shakes her head and closes her eyes. “It’s dumb.”

“I doubt that.” It’s the mildest version of any of the things I’d like to say to her. She’s so funny and huge-hearted and creative and clever. There’s nothing dumb about her. She may have high walls around her, but the more I get to know who Zoe really is, the more I love the tender heart of her — the girl who is devastated by her brother leaving, the one who is willing to let me push her outside her comfort zone, the one who seems to like me in spite of my epic weirdness.

The person who sees me.

“Okay,” she says. “So I have this theory about being adopted. I call it the Theory of Original Defectiveness. Because I was immediately deemed not worth keeping.” She pauses. “I mean, nothing is more fierce than a mother’s instinct to protect her young, right? Darwin and everything?”

“Right,” I say. My mom’s instincts are sometimes so fierce, they don’t even make sense.

A pained look crosses her face. “So there was something about me that was not worthy of that. Of being kept. Hence, original defectiveness.”

The thought of Zoe being defective or someone not wanting her doesn’t compute. I’m probably the one who should worry about defectiveness — my dad knew me well and still chose to leave me behind.

“So I try to make up for it,” she continues. “I try to be good. And good daughters don’t swear. They don’t cause trouble. They’re not ungrateful; they don’t get bad grades; they don’t wish for things they can’t have.”

“Ah,” I say. Now I get it. With the understanding comes a wave of sadness.