The explanations of thirty primary pupils for changes in matter were recorded through individual, semi-structured interviews. The analysis of data pointed to the construction of a system for classifying pupils' explanations of changes in matter. A parallel analysis of data focused on the identification and interpretation of associations between the explanatory categories resulting from the classification, and certain variables of the context, that is (a) category of change to be explained, and (b) familiarity with the system about which they were being questioned. The results of the two phases of the analysis were combined with the findings of a previous study, to produce the formation of a limited number of pupils' models concerning matter, changes in matter and their explanations. Pupils' models involve a variety of conceptions which could be attributed to the presence of a conceptual obstacle: that is the "continuous and static conception of matter." The definition of teaching objectives in relation to obstacles ("obstacles as objectives") can bring together pupils' conceptions with concepts to be built and consequently to contribute to the design and evaluation of constructivist teaching and appropriate material.