In this article, the authors describe a series of activities they have used with middle-level students. The first set of lessons explores density through the layering of liquids. In the second set, they use some of the same liquids to explore the density of solids. The third set investigates how temperature affects the density of materials--primary liquids. The fourth set leads to quantitative measurement of the density of solids. Concept development, problem solving, design, measurement, and quantitative activities are interwoven throughout these lessons. Each set of lessons is designed to conform to one full cycle of the 5E (Engage, Explore, Explain, Elaborate, and Evaluate) learning model. Ideally these lessons should be done in order, but teachers may choose one or two for their class as appropriate. The activities within each lesson need to be done in order, however. (Contains 6 figures, 1 resource, and 1 online resource.)