The purpose of this study was to characterize fourth grade students’ mental models of a closed circuit and examine to what extent students’ mental models were influenced by engagement in an engineering design task. In an effort to capture students’ pre and post understandings of a closed circuit, an open response question and follow up interview was administered before and after students’ engagement in a door alarm design task. Results indicated an array of students’ different mental models. Additional analysis indicated that students developed a more informed and accurate understanding of simple circuits as a result of participating in a design task. Implications of this study suggest that engineering design may facilitate students’ science conceptual understanding.