The purpose of this research project was to study how students in the first years of elementary school (children from 7 to 10 years of age) are initiated into the construction of explanations of physical phenomena in the teaching of science. With this purpose in mind, we organized classes based on the proposition of investigative problems, where children, working in groups, could solve problems by raising and testing their own hypotheses. They would then attempt, by means of general discussion organized by the teacher, to discuss how each problem was solved and why it worked. We videotaped a series of classes in which the students solved 15 different investigative problems. We also analysed the teacher/student interactions that took place (in this paper, we present data on two of these classes). Based on our data we found that students construct their own causal explanations by following a sequence of stages that includes the appearance of novelties. We also discuss how our data relate to the teacher's role in the classroom and to the organization of science teaching at this level.