The ability to identify promising ideas is an important but obscure and undeveloped aspect of knowledge building. The goal of this research was to examine the extent to which young students can make promisingness judgments and, as a result, engage in more effective knowledge building. Toward this end we embedded a design experiment in a Grade 3 classroom. In this experiment students were engaged in discussion and reflection of the concept of promisingness and used a Promising Ideas tool to identify promising ideas in their written online discourse. They used the tool for two refinements of idea selections to focus ongoing community dialogue. Results suggest that students as young as 8 years of age can make promisingness judgments that facilitate knowledge advancement in their work. These results inform future work in classroom interventions and tool development to promote promisingness judgments in collaborative knowledge building.