The purpose of the present study was to design knowledge building classrooms where a community of learners works collaboratively toward deep understanding. This paper presents our design of learning activities and its impact on students' science learning. Using design research as a methodological framework, we collected and analyzed multifaceted data for in-depth understanding of our research problems. Overall, we found positive impacts showing improvement of student understanding in the course of reflective thinking and progressive inquiry. Pretest and posttest results suggest that our instruction helped increase students' conceptual knowledge, and that high-achieving learners gained more benefits. In the Knowledge Forum environment, students were able to improve their initial understanding by reading, connecting, and questioning other ideas. We conclude the paper with implications for design challenges.