This paper examines what is important about talk between learners during school science and, having identified this, suggests how we can ensure that what we consider important happens. By looking at the interaction between teachers and learners talking about science, it is possible to indicate ways in which learners can be helped to continue this learning conversation with one another when teacher support is withdrawn. Strategies for teaching and learning are examined. The paper reports on the findings of a research project designed to teach children how to negotiate their ideas about science concepts through rational dialogue. Children's development of scientific concepts in classrooms is undertaken through structured activity and mediated through oral language. Children must move forward simultaneously in their use of specialized vocabulary and in their understanding of current scientific explanations, models and ideas. New language and new ways of using language are learned by doing, which means for children, primarily speaking and listening. Children's understanding of science can benefit from teaching them to understand that spoken language is a powerful tool for thinking together.