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Bullies beware! In the town of Blue Wing, Minn., on the banks of the Mississippi River, 12-year-old Claire watches with fascination as her mean cousin Duke grows a rhinoceros horn after throwing Claire and her pet turtle off a bridge. Debut novelist Helgerson, through the heroine's first-person narration, does a remarkable job of making the magical goings-on seem completely plausible. The old woman who rescues Claire from the river, for instance, explains that only bullies who fall into that particular section of the river will grow a horn (Duke winds up getting tossed into the water himself); each month, rock trolls put a potion in the river to cause the physical change, then collect the bullies. The only way to reverse the spell is to perform a genuine act of kindness—and promptly, or the afflicted will transform into a wrinkly rhinoceros. It turns out that these horns have plagued Claire's family for generations; even the medical community is aware of these "rivery" effects. As with most memorable enchanted tales, it's up to the children to reverse the curse, and Claire asks in exchange for a good deed of her own. This likable heroine, full of spunk and tenacity, unspools her lively account at a quick pace, and quirky characters such as Bodacious Deepthink, the Great Rock Troll, add plenty of pizzazz. Nicoletti's half-tone chapter openers combine meticulous detail with fuzzy shadings, contributing to this charming book's paradoxical success as a thoroughly believable fantasy in which magic and the real world co-exist. Ages 9-12. (Sept.)