The role of performance assessments in making cognitive activity apparent to teachers and students was studied through descriptions of the cognitive activity of 31 fifth-grade students. Differences while carrying out a science performance assessment reveal critical differences between those who think and reason well with their knowledge of electrical circuits and those who do not. Differences in quality of explanations, adequacy of problem representation, appropriateness of solution strategies, and frequency and flexibility of self-monitoring indicate more or less effective learning of the subject matter. Awareness of and attention to these cognitive characteristics of competent performance in an assessment situation provide teachers the necessary feedback to construct classroom environments that encourage reasoning and knowledge integration. In this way, performance assessments not only evaluate student performance but suggest changes in instructional practice to support effective learning in the elementary science classroom. (Contains 1 figure, 1 table, and 23 references.) (SLD)