We address the question "how does conceptual change take place in the science classroom?" from the perspective of Giere's cognitive science model. The research work involved a group of 21 students from the first year of the Spanish "Bachillerato" or upper secondary school (17 year olds) in a public high school in Barcelona. Once the students' mental representations about respiration were known, the study of their evolution started. Academic performance of students was chosen as the criterion to select the subjects: one with high academic performance, two with medium academic performance, and one with low academic performance. The analysis refers to two of the cases that were analyzed in depth: one with high academic performance and the other with low academic performance. The aim of selecting only these two cases was to achieve a better contrast among them. We found that, in some subjects, there was co‐existence of various stable and useful explicative models, with reference to different situations. Other students used undifferentiated sets of ideas instead of explicative models; they are students who show little critical and abstractive capacity. In our study, those students who used models drew more coherent and cohesive explanations, and those models were used more specifically according to the various contexts. We observed a gradual process of conceptual evolution, in which, differentiation among the models that constitute the model family, as well as internal enrichment of each of those specific models took place at the same time. From this perspective, conceptual evolution is not reached by the juxtaposition of ideas belonging to different explicative models, but by the identification of similarities and differences among the different models that attempt to explain a given reality.