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² Smith, C., Carey, S., & Wiser, M., (1985). On differentiation: A case study of the concepts of size, weight, and density. Cognition, 21, 177-237.

³ Houghton, C., Record, K., Bell, B., & Grotzer, T.A. (2000, April). Conceptualizing density with a relational systemic model. Paper presented at the Annual Conference of the National Association for Research in Science Teaching (NARST), New Orleans, LA. Kohn, A. S. (1993). Preschooler's reasoning about density: Will it float? Child Development 64(6), 1637-50.
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⁶ Smith, C., Maclin, D., Grosslight, L., & Davis, H. (1997). Teaching for understanding: A study of students' pre-instruction theories of matter and a comparison of the effectiveness of two approaches to teaching about matter and density. Cognition and Instruction, 15(3) 317-393.

⁷ Grotzer, T. A. (1993). Children's understanding of complex causal relationships in natural systems. Unpublished doctoral dissertation, Harvard University, Cambridge.
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⁸ Andersson, B. (1986). The experiential gestalt of causation: A common core to pupils' preconceptions in science. European Journal of Science Education, 8(2), 155-171.
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