“Fine,” I say, with the same attitude as hers. “Keep it.” While Mattie sorts her supplies, I open the transit app on my phone. I’ve taken the bus to the mall a couple times with Gaby, who knows all about the buses. But I’ve never done it by myself. “We’ll go next Friday after school,” I tell Mattie. I don’t explain that that’s Tristan weekend to go to his dad’s. That it will be easier for me to leave if I don’t have to worry about my brother.

I scan through the schedules and the maps. “Bad news, Mattie,” I say at last. “Transit doesn’t go all the way to Northern Madrone College. We can get about halfway there.” I lean over and show her the map. She grabs the phone from my hands.

“What?” Panic flashes across her face. “That’s terrible! What are we going to do?” She scrolls around on the app. And then she sighs. “No, Mercedes, look. We’ll just have to switch buses. See?”

I grab my phone back. She’s right. We’ll have to transfer. No problem. I’ve heard people asking for transfers. We’ll be able to do it. I hope.

“My mom’s coming to pick me up in ten minutes. And we’re not even close to ready.”

“Just tell her you need to stay later. You can sleep over if you want.” I gesture at the backpack. “You have plenty of stuff.”

“But I don’t have my toothbrush, remember?”

“I have extras. Besides, haven’t you ever skipped a night?”

“I guess.” Mattie looks unsure. She digs in her backpack again and pulls out her phone. She doesn’t say anything, just scrolls. I catch a glimpse of her photos. Pictures of her smiling with other kids. “I haven’t had a sleepover since July. Me and my friend Mai.”

She sounds sad and I want to shove her sadness away. “Look,” I snap, “it doesn’t matter to me what you do. Just decide.”

“Are you sure?”

I’m not sure I want Mattie Gómez sleeping over, no. On the other hand, she is my half sister. Maybe it’ll be okay.

“Thanks.” After some tapping on her phone, she says, “Hello? How was the movie?” There’s a pause. “I was wondering if I can stay overnight.” Another pause. “Yes, her nanny is here. It’s fine with her.” Mattie looks at me as if to say, Is it? I nod. “It’s the same number. You can call her. Okay, I will. I promise. I love you.”

“Here,” I say when she tosses aside her phone. “Have an iced tea.” I throw her the bottle.

Once we’ve devoured half the snacks, we divide the job of planning our escape—and our exit strategy. She studies the campus map and I look through the bus schedules.