Constructivist views of learning in science suggest that learners can only make sense of new situations in terms of their existing understanding . Prior knowledge is used by learners to interpret observations; meaning is constructed by individuals in a process of adding to or modifying their existing ideas (Driver, 1983; Osborne and Freyberg, 1985; Scott, 1987).
The implications of such a view are that teachers need to find out the learners' ideas in order to take these into account in their teaching. Teachers then need to provide experiences which challenge the learners' current understanding in order to help them restructure their ideas. One example of a teaching and learning sequence which describes this kind of approach is shown in Appendix 1 (Driver and Oldham, 1986).
Constructivist perspectives have had a significant impact on recent research in science education in the UK and elsewhere. Much recent research has been concerned with finding out the ideas which learners typically hold in order to inform teaching. Many science teacher educators are persuaded of the value of a constructivist viewpoint and actively promote a constructivist philosophy. As a consequence it is now commonplace to find that teachers have been exposed to constructivist ideas, have a commitment to constructivist principles and have made some attempt to modify their practice to take these principles into account. Recently curriculum materials have begun to emerge which adopt an explicit constructivist approach, Nuffield Primary Science (1993a) being a prominent example.
Bentley and Watts (1991) claim that there is a distinction between what they characterise as the "strong" theoretical version of what is involved in constructivism and the "weak" version of constructivism in practice which many teachers implement in their classrooms. It may be that teachers have misunderstood what the researchers are saying and that any modification to their practice is based on a misinterpretation of constructivist principles.
Alternatively the distinction claimed by Bentley and Watts may be evidence of a more significant issue. Our belief is that the model of constructivism generally put forward is one which is based largely on methods which work for researchers rather than methods which work for teachers in typical classrooms. The discrepancy between the version put forward by researchers and the version applied in the classroom may therefore be indicative of a realistic response by teachers, who may be committed to a constructivist approach in principle but have to find ways of making this manageable in practice. We believe that a number of significant issues about how to make constructivism applicable to classrooms have not yet been addressed by the various research groups.
Issues relating to constructivist approaches to teaching and learning
One issue is that of the separation of the phases of the constructivist teaching and learning sequence. Although it is recognised that the elicitation phase may provoke learners to reconsider their thinking (Needham, 1987), the phases are generally described as separate and distinct (Driver and Oldham 1986; Needham, 1987; Scott, 1987). Teachers commonly perceive these phases as separate, and Nuffield Primary Science (1993b) suggests that teachers may have difficulty in switching from one phase to the other. Curriculum materials produced by Leeds (1992) describe how teachers will find it easier to plan activities after the learner's ideas have been identified. However as a model for teaching this separation is not helpful. The practicalities of timing and planning for teaching make this model, which works well as part of a research programme, very difficult to implement. The separation of the two phases also means that the purpose of the elicitation phase is only evident to the teacher and not to the learner.
Precisely how the learner's ideas can be taken into account in planning suitable restructuring activities is also a cause for concern. In principle the suggestion that teachers should plan activities on the basis o