The development of the concept of matter was explored in children aged 3-13. Eighty four children were asked to classify four sets of objects and matter and to explain their classifications during interviews. Younger children tended to classify using a mixture of extensive properties (properties of objects) and intensive properties (properties of matter), whereas older children (above 9 years old) tended to use intensive properties most of the time. The effect of using sets of objects and matter with different characteristics was explored. The different sets of objects and matter consisted of ones made of different substances, of various shapes and particle size, and of different states of matter. The ways in which children classified were analyzed from a Piagetian perspective, which hypothesizes that the epigenesis of concepts takes place through children acting on the world. Through their actions children gradually develop more elaborated schema which enable them to distinguish between extensive and intensive properties and hence between object and matter. (Contains 4 tables and 2 figures.)