In the present article, we argue that the conceptual change approach to learning can apply in the case of mathematics, taking into consideration the particular nature of mathematical knowledge and the neurobiological bases of mathematical cognition. In the empirical study that is reported in this article, we investigated ninth graders' understanding of algebraic and structural properties of rational numbers, from a conceptual change perspective. We make the point that understanding rational numbers is not indiscriminately difficult. We show that prior knowledge about natural numbers supports students dealing with algebraic properties of rational numbers, while the idea of discreteness is a fundamental presupposition, which constrains students' understanding of density.