This inquiry-based curricular unit introduces middle school students to the concept of density through five hands-on experiments. In Activity 1, they handle five identical boxes, each filled with a material of differing density, to get a feel for the relationship among mass, volume, and density. Next, they devise methods to figure out the density of solid objects by using water displacement. Finally, they use their knowledge to determine how to calculate density without being given a formula. Activities come with learning objectives, student guides, teacher content support, and ideas for assessment. All are aligned to national math and science standards. Teach Engineering is an NSF-funded Pathway developed to provide high-quality experiential learning materials for K-12 classrooms.