Models are central to the practice and teaching of science. Yet people often fail to grasp how scientific models explain their observations of the world. Realizing the explanatory power of a model may require aligning its relational structure to that of the observable phenomena. The present research tested whether "relational scaffolding"--guided comparisons between observable and modeled events--enhances children's understanding of scientific models. We tested relational scaffolding during instruction about the day-night cycle, a topic that involves relating "Earth-based" observations to a "space-based" model of Earth rotation. Experiment 1 found that 3rd graders (N = 108) learned more from instruction that incorporated relational scaffolding. Experiment 2 (N = 99) found that guided comparison--not merely viewing observable and modeled events--is a critical component of relational scaffolding, especially for children with low initial knowledge. Relational scaffolding could be applied broadly to assist the many students who struggle with science. [This is the in press version of an article to be published in "Psychological Science."]