Even though density is taught in middle school, high school students often struggle to understand that the density of a substance is consistent regardless of amount. This is because many high school students know density = mass/volume, but do not have the conceptual understanding necessary to explain density-related phenomena. The scaffolded approach to teaching density--as the authors have described in a companion article in this issue, "Sinking In," and in this article--started with students' conceptions of mass and volume and added a layer of complexity with each activity. While this approach can take up to several days, developing a deep understanding of density and kinetic molecular theory is a critical prerequisite for understanding numerous high school disciplinary core ideas such as chemical reactions, rates, and equilibrium (HS-PS1.A and PS1.B); structure and properties of matter (HS-PS1.A), the meaning of energy (HS-PS.A), Earth materials and systems (HS-ESS2.A); plate tectonics (HS-ESS2.B); and weather and climate (ESS2.D). Students also develop understanding of science and engineering practices like obtaining, evaluating, and communicating information and developing and using models, along with crosscutting concepts like energy and matter, and cause and effect. Deeply learning scientific ideas does take time, but the time is well spent. [For the companion article, "Sinking In," see EJ1120903.]