If teachers decide to introduce the idea that the behaviour of matter ("stuff") can be explained by thinking about it as being made up of particles that exhibit particular properties, then they need to be aware of the learning difficulties involved. Before using this approach, therefore, more immediate consideration needs to be directed to children's understanding of related, and possibly prerequisite, concepts. A small number of studies indicate that some children can understand some aspects of the particulate (or particle) nature of matter. Primary children use "atoms" and "molecules" in everyday language, and Leisten (1995) believes that they can understand these concepts. In this article, the author considers how to approach teaching about atoms, molecules and substances in the primary phase.