Elementary teachers are navigating a curriculum landscape in which engineering activities now exist alongside science. Teacher education should prepare future elementary teachers to be critical curators of curriculum resources, which includes appraising the extent to which engineering and science activities authentically reflect those fields. To date, though, research has not examined how teachers think about what makes a lesson "engineering" rather than "science." In this study, we examine how a group of elementary pre-service teachers (PSTs) determined the extent to which a set of classroom activities represented engineering and/or science instruction. PSTs analyzed a set of lessons before and after completing an online module that was designed to expand their understanding of the nature of engineering. The module gave PSTs examples of real-world engineering work, drew distinctions between science and engineering, and modeled how those ideas can be applied to instructional decision-making. We found that PSTs identified lessons as "science" when students were conducting investigations and collecting data while learning disciplinary content. In contrast, PSTs saw engineering activities as ones that involved students constructing physical products while solving practical problems. After completing the online module, PSTs placed less emphasis on the "making" of products and more on cognitive aspects of engineering problem-solving, such as testing ideas and justifying design decisions. Our results indicate how teachers' understanding of the natures of engineering and science can play a role in how they think about what makes an activity "engineering" and/or "science."