This paper aims to study the relationship between questioning and the quality of knowledge constructed in a CSCL context. In previous studies, different units of analysis are employed and the patterns of relationships are not consistent for different age groups. The analytic units of a thread, an individual, and a group are employed in this study, and two grades of students (grade 6 and grade 10) are investigated. At the thread level, we aim to examine to what extent questioning can be a mechanism for knowledge advancement, and we find that the tenth-graders are capable to advance knowledge through questioning, while for the sixth-graders, more facilitation seems to be needed. At the individual level, we find that for the sixth-graders, those asking good questions are likely to express high-level explanations, suggesting that individual competence plays an important role in advancing knowledge among these younger students. For the tenth-graders, a "division of labor" is observed that good questions are generated by some students while high-level explanations by others. At the group level, a high and positive correlation is found for both grades. The findings suggest further research is needed to examine other possible mechanisms for knowledge advancement in online collaborative discourse.