Sarah, who is eight, is filled with excitement as her family plans to travel by wagon from Missouri to Oregon; her best friend, Almira Ann, and her family are moving, too. The girls share everything, including a birthday, on July 4th. Sarah chafes under the constraints of keeping to the wagon as they travel, and finds far more comfort in her father’s dreaminess and her mother’s cheer than in her grandmother’s efforts to keep the family on task and together. A series of vignettes inspired by actual historical incidents brightens this story: the cow rescued by drilling holes in its horns to pull it out of the mud; Lakota coming by to trade items; Sarah’s climbing Chimney Rock herself to scrawl her and Almira Ann’s names there. The impulsive event that affords the book its title happens late, long enough along for readers to feel both the hard work and sacrifice of the journey, as well as its undeniable romance. Sarah’s headstrong personality comes across clearly, as do her efforts to set things right when she’s done wrong. (b&w illustrations, not seen) (map) (Fiction. 9-12)