This paper focuses on the evidence of learning in children engaged in small-group discussion with a teacher. The children are considered to have learned if they show progress in the explanations they make, the language they use in the dialogue, or the understanding they show of the dialogue. Motives of the participants in this dialogue are analyzed and the teaching-learning process is considered to be the resolution of productive misunderstandings between everyday notions and scientific notions. The concluding discussion draws implications and raises questions about the role of the teacher in scientific dialogue. It is proposed that teachers might understand their role partly as a learner investigating the children's understandings, and partly as a guide who validates productive ideas and introduces scientific language to help the dialogue advance. Strengths of activity theory as an analytical tool for the classroom discussed include action-on-dialogue as a unit of analysis and activity system for the analysis of a community in dialogue. Contains 26 references. (Author/PVD)