An important goal of the current reform movement in science education is to promote scientific literacy in the United States, and scientific inquiry is at its heart. However, the National Science Education Standards clearly indicate that to promote inquiry, more emphasis should be placed on "science as argument and explanation" rather than on science as "explanation and argument." They also call for scientific argumentation to play a more central role in the teaching and learning of science (NRC 1996). To that end, this article describes a multiple-day, inquiry-based lesson about the phases of the Moon that engages students in scientific research and argumentation. (Contains 6 figures.)