“How is that even possible?” I say out loud. The photo I have of Francisco Gómez is in a black metal frame on my dresser. It’s always been in my room somewhere, for as long as I can remember. I packed it in a box in Minnesota and unpacked it again in California. I walk past it every day, and mostly I don’t even notice it. Like the way you know you have an elbow, but you don’t think about it much. It’s just there. Until you bang it on the kitchen counter, then you remember. The photo on my dresser is just the face of someone who is my father. The father I don’t know. A zing goes up my spine. My mom. My father. “Wait.” A terrible, horrible thought occurs to me. “My parents had two kids?”

Mercedes rolls her eyes. “People can have the same dad and different moms.”

Oh. Right. The zing zings away.

“I mean, my brother and I have different dads,” Mercedes explains. “Tristan is seven and”—she holds up a dark-brown chunk of her flat-ironed hair—“he has blond hair, blue eyes.”

She flips her hair and slings her backpack over her shoulder. “Thanks, Mrs. Ellingham!” Mercedes calls as she heads for the door.

“Wait!” I call to Mercedes, grabbing my stuff. She’s leaving? Now? I need more time. The clock above the whiteboard ticks toward four o’clock. My mom will be here in a few minutes.

“Anytime, girls.” Mrs. Ellingham smiles like today is a regular day, not a day when a total stranger has a picture of your father in her notebook.

“Listen, Mattie,” Mercedes says when we’re outside the classroom. “We have different moms. And we have the same dad.” She says it so matter-of-factly.

“But—”

She speed walks toward the front entrance, her sneakers silent on the tiles of the breezeway. The sunlight churns through the evening fog like it’s watching us, making strange shadows across the buildings. I run after her. I feel so out of place. I shouldn’t be here. In this school. I shouldn’t be in California. I shouldn’t be chasing a girl who looks like me and says we have the same father.

And then I realize something. I stop. I’m panting.

“Wait!” I call again. She doesn’t stop. “Mercedes, wait!” I yell. I run past the red-and-yellow lockers and the closed classroom doors and the custodian emptying trash cans. I catch up to her and grab the strap of her bag just as we reach the driveway. A girl I recognize from math class climbs in a two-door car with a rusty hood.

“If we have the same father,” I say after the rusty car pulls away, “does that mean—?” I gasp. “That means we’re sisters?”