According to some authors, the ability to think "in historical terms" is mainly based on the process of explanation and explaining is itself a social activity since people explain something to someone in order to obtain an effect. Primary school can be an important context for the development of this ability: it offers some educational situations, such as discussion between peers, that are particularly useful for facilitating children's explanatory reasoning. This research aims at understanding how discourse about historical topics moves and develops into a specific interactional context: the small-group activity. Ten small autonomous groups of 8-9 years old children (belonging to two different Italian primary classrooms) were invited to discuss an iconographical document. They had to answer to some questions concerning the document itself but they had to reach an agreement before writing the answers. Qualitative analysis of interaction within groups shows that "peer interaction" is an important tool for the development of explanatory reasoning: the activity of working together with a common aim enables children to discuss, cooperate and, sometimes, disagree with peers. In situations like these, children are able to "think together", make hypotheses and negotiate historical meanings, even though they do not feel fully confident about their own ideas and opinions.