The purposes of this study were to: (1) investigate the process of development of young children's ideas when (and if) they appropriate science from the conceptual area (i.e., light) during whole-class and small-group interactions; and (2) advance understanding of relationships between a collaboratively designed learning and teaching model and children's developing scientific knowledge. This paper discusses children's learning in a Year 1/2 class. Children cooperated with their teacher's skillful modeling of how to conduct fruitful discussion. The high engagement of children in class discussions was revealed by their ever-lengthening and increasingly fluent contributions. Children expressed complex ideas while their classmates listened intently and interacted directly with the contributor, unmodulated by their teacher. The informal inquiry sessions where children could investigate their own questions or those generated in class discussion stimulated further thinking about the learning-model context and provided material for reflection on learning. Mediational means for the development of children's thinking can be attributed to child, teacher, and parental characteristics grounded in their middle-class culture and values, in the design of the learning and teaching model, and in the use children made of their personal learning journals. Contains 51 references. (Author/PVD)