This paper presents the results of a study of the cognitive strategies in which ninth-grade science students engaged as they used a learner-centered dynamic modeling tool (called Model-It) to make original models based upon stream ecosystem scenarios. The research questions were: (1) In what Cognitive Strategies for Modeling (analyzing, reasoning, synthesizing, testing/debugging, and explaining) do ninth-grade science students engage as they create dynamic models? and (2) What are characteristics and qualities of the Cognitive Strategies for Modeling in which they engaged? Sixteen representative ninth-grade students from a public school in a midwestern college town were paired to create models on the software. Most pairs of students engaged in an analysis of appropriate objects and factors for their model and in relational reasoning about those factors. Most were able to synthesize a working model employing a range of strategies, and most attempted to articulate explanations for their relationships. Most tested their model but only a few persisted in debugging to fine-tune their model's behavior to match their expectations. These findings indicate that creating dynamic models has great potential for use in classrooms to engage students in thought about the science content they are supposed to learn. (PVD)