Guided inquiry is an instructional technique that requires students to answer a teacher-proposed research question, design an investigation, collect and analyze data, and then develop a conclusion (Bell, Smetana, and Binns 2005; NRC 2000). In this article, the authors describe a guided-inquiry lesson developed through the lesson-study process (Fernandez 2002; Lewis 2002). This lesson helps students understand the concepts of diffusion and osmosis as they relate to cell function (Content Standard C; NRC 1996) and develop the abilities necessary to conduct scientific inquiry (Content Standard A; NRC 1996).