Explanation as a genre may support children’s reasoning and understanding particularly effectively. In this study, 20 fourth graders were given the task of explaining the effects of a pollutant on an ecosystem to third graders. Before writing, they completed a commercially developed science unit, instruction in reading and writing an explanation, and text reading. An analysis of their writings revealed that all children used rhetorical devices to connect with third-grade readers. Sixteen children synthesized text content with personal experiences to compose subexplanations that reported information, gave examples, and presented scenarios and that were logically ordered to enhance reader understanding. Nine of these children explicitly used the scientific model to explain phenomena. Outcomes suggested that writing explanations supported children’s reasoning about and understanding of an important scientific model.