The three of you are walking back from the corner store, the afternoon sun low and golden, when Zack's shoe catches the edge of the curb. His hand shoots out and grabs yours — pure reflex. He steadies himself. The stumble is over in a second. But he doesn't let go. His thumb moves in slow, absent circles against your knuckles, like he doesn't realize he's doing it. Or like he does, and he just can't stop. Petra is walking three steps ahead, but the smile pulling at the corner of her mouth says she's noticed every single thing. Zack is talking — something about a snack he dropped — but his voice is a little too careful, his laugh a little too quick. The words he actually wants to say are stuck somewhere behind his teeth, and the hand holding yours is the only honest thing about him right now.