Abstract
This study examined the reasoning discourse of two pairs of ninth grade students in the Midwestern region of the United States. The purposes of the study were to elucidate specific types of reasoning strategies that students use in ordinary classroom activities and compare the reasoning strategies used in an activity that required little domain‐specific knowledge to one that required extensive domain‐specific knowledge. Interpretive techniques were used to explore the students’ thinking represented in discourse, writing and interviews. Results indicated that successful model formulation forboth activities involved: recognizing the tenative nature of existing models; identifyinglaboratory observations as evidence and using evidence to modify existing models; and coordinating all mutually consistent knowledge propositions into a coherent new model. Forthe activity requiring substantial conceptual knowledge, both the generation of relevantknowledge propositions and their coordination with laboratory evidence were important in constructing an accurate scientific model.