Earth magic

Thank you for watching my earth child. In an emergency, please call me. . . . So begins the mysterious note Polly and Sam discover in the woods near their grandparents' farm. There's no child in sight, but they do find a most unusual rock. Polly and Sam drag it home, but the strange-looking stone won't stay put! Before long, it is clear that something magical is happening to them again!From the Trade Paperback edition.