A previous study (Wallace, Yang, Hand, & Hohenshell, 2001) indicated that seventh-grade life science students using a learning tool known as the Science Writing Heuristic (SWH) performed significantly better on conceptual test questions than did a control group. In the present study, the researcher studied more deeply how students utilized a variety of knowledge sources while engaged in the SWH, including textbook, teacher-led discussions, laboratory activities, peer group discussion, and writing (including their cognitive mechanisms and the nature of their written explanations). Six case students were selected based on a range of high to low achievement according to grades. An interpretive analysis of interview and document data was conducted. Of the 6 students, 3 relied on firsthand observations from laboratory activities as their major source of understanding; these students used listening, explaining, and writing most frequently. One student relied solely on textbook and teacher statements and actively rejected laboratory observations, relying primarily on reading and synthesis. Two students integrated laboratory observations with canonical information found in the textbook and other reading sources. They were able to bridge between the different epistemological bases for firsthand observations and authoritative text and blended these into rich and detailed explanations for biology concepts.