Idea improvement, supported by sustained efforts at collective knowledge advancement, is the essence of knowledge building and the discourse that supports it. This study explores the dynamics and nature of knowledge building discourse in a grade 3, knowledge building community, using the discourse medium---Knowledge Forum---to support them in making their ideas public and objects for continual improvement. Their collaborative group discourse in this environment is analysed in light of knowledge building principles: real ideas, authentic problems; idea diversity and improvable ideas; and rise above. Implications for knowledge building theory and practice are discussed.