Teachers working with Key Stage pupils might, for instance, focus on the subject content of science and develop science skills from these areas of experience. This product centred approach could, for example, give rise to oral explanations and demonstrations of scientific knowledge, and, from time to time, practical activities designed to provide direct experience of phenomena with opportunities to explore and investigate these phenomena. In providing a conceptual structure to help the learner build a functional mental representation, the teacher highlights what is relevant and the nature of the relationships between the elements. For example, the teacher might explain the compressibility of air in a bicycle pump by describing it as dispersed particles which may be brought closer or else by comparing it with the behaviour of a spring. In contrast, teachers might focus on the processes of science and develop scientific conceptual understanding from it. This process-centred approach could, for instance, offer the children experiments and investigations as starting points for acquiring conceptual knowledge with little or no direct teaching of concepts. In this case a conceptual structure is withheld. The onus is on the children to recall or construct a functional mental representation without reference to a teachers' description of one. Pupils might infer relationships in the topic under study and may be given an opportunity to test and revise their ideas. Of course, other teachers might focus on a combination of these two approaches and develop scientific skills and conceptual understanding from in this combination. This mixed approach could be a balance or, perhaps, a compromise, between a product-centred and a process-centred approach, in which the teacher provides a partial conceptual structure and leaves the remainder for children to construct by inferring, hypothesising, or testing their ideas. It could encourage lessons where children do investigations with some features already identified by the teacher, and with some conceptual knowledge about the subject that enables them to appreciate the purpose of the activity. In contrast, it could encourage lessons without a clear purpose which mixed different types of activity, but did not develop either conceptual or procedure understanding exclusively.