This pilot study used a lesson plan study to examine the thinking of preservice teachers about the concept of density, part of almost every school science curriculum, and the teaching of that concept to middle grade students. Subjects in the first study year included seven preservice teachers grouped into three clusters to accomplish the lesson planning task. In the second year, a subset of three students worked together to prepare the lesson plan. Data analysis focused on two facets of understanding density, intuitive and mathematical. Findings suggest that preservice teachers may possess discrete understandings about science content that they have not yet connected in a way that makes sense for them. Probing questions may be a tool to enable them to construct more useful holistic concepts for themselves. Study findings also suggest that the lesson plan study method is a promising strategy for examining the understandings of preservice teachers. Two appendixes contain the lesson planning instructions for both study years. (Contains 22 references.) (SLD)