[Abstract] This paper presents the design of Idea Thread Mapper (ITM), a timeline-based collective knowledge-mapping tool that interoperates with Knowledge Forum and potentially other learning platforms. Students engage in collaborative, metacognitive conversations to identify "juicy topics" that have emerged from their knowledge building discourse, as their communal focuses, and reviews ideas addressing each focus as a line of inquiry--an idea thread. A study was conducted in a Grade 3 classroom to foster sustained knowledge building through ITM-aided collaborative reflection. Analyses of interviews, classroom videos, online discourse, and ITM data elaborate the processes and benefits of ITM-aided collaborative reflection to foster student awareness of their collective knowledge and collaborative, sustained efforts to advance it.