What literacy device explaines why the kids were really gathering stone for.

The children assembled first, of course. School was recently over for the summer, and the feeling of liberty sat uneasily on most of them; they tended to gather together quietly for a while before they broke into boisterous play, and their talk was still of the classroom and the teacher, of books and reprimands. Bobby Martin had already stuffed his pockets full of stones, and the other boys soon followed his example, selecting the smoothest and roundest stones; Bobby and Harry Jones and Dickie Delacroix—the villagers pronounced this name “Dellacroy”—eventually made a great pile of stones in one corner of the square and guarded it against the raids of the other boys. The girls stood aside, talking among themselves, looking over their shoulders at the boys, and the very small children rolled in the dust or clung to the hands of their older brothers or sisters.