Current reform efforts in science education in the United States call for students to learn science through the integration of science and engineering practices. Studies have examined the effect of engineering design on students' understanding of engineering, technology, and science concepts. However, the majority of studies emphasize the accuracy of students' scientific thinking instead of what students' conceptions are. The aim of this study was to examine elementary school students' conceptions of sun-Earth relationships as a result of engaging in an engineering design-based science task. Two independent fifth grade classrooms were identified. Each classroom teacher had 2 groups of students: 1 group engaged in traditional science lessons (control) and 1 group engaged in engineering design-based science lessons (treatment). Data were collected via multiple choice knowledge assessments, a draw-and-explain item, and semi-structured interviews designed to elicit students' working mental models of the relationship between the sun and Earth. Results indicated a range of five different mental models expressed by students in both the control and treatment groups. These findings suggest that students still harbor alternate conceptions and possibly conflicting ideas regarding various sun-Earth relationships. If teachers are expected to implement science and engineering practices, attention must be given to not only what students' misconceptions are but, more importantly, how best to implement design-based science lessons that facilitate students' application and understanding of related science concepts.