The relationship between teaching and learning is a most central issue in education research, but it is yet to be fully understood. Variation Theory provides an explanatory account for teaching and learning and offers a specific view on what it means to learn: to discriminate and to discern critical aspects of the object of learning. This paper explores the use of Variation Theory in a case study of two lessons on density in a secondary science classroom. By explicating what was made possible to learn and what was learned this study demonstrates potentials and limitations offered by Variation Theory in enabling a better understanding of the teaching learning relationship.