The main goal of this paper is to show how Vasily Davydov's powerful ideas about the nature of mathematical thinking and learning can transform the teaching and learning of additive word problem solving. The name Vasily Davydov is well known in the field of mathematics education in Russia. However, the transformative value of Davydov's theoretical work in this field has not yet been fully recognised by the larger international community. In this article, I use Davydov's vision of mathematics as the study of quantitative relationships--the vision underlying the relational paradigm in teaching and learning of mathematics. I use the example of one research project to demonstrate how the relational paradigm changes and reforms teaching practices. I discuss the teaching approach, developed within this paradigm, as well as the learning outcomes demonstrating the transformative power of Davydov's ideas.