There is a substantial body of knowledge on the importance of language for learning in general, and for learning mathematics in particular. Hence, language skills and collaborative learning are emphasised in the Norwegian curriculum. Even so, we have few studies on what supports and what impedes mathematical progress in authentic learning situations. In this article, we investigate contrasting dialogues between two pairs of eight-year-old pupils solving mathematical tasks. The analysis in our video-based study shows that both communication skills and use of tools have a profound impact on third graders' potential to solve tasks as a joint enterprise.