This paper reports design experiments on two Japanese elementary science lesson units in a sixth-grade classroom supported by computer support for collaborative learning (CSCL) technology as a collaborative reflection tool. We took different approaches in the experiments depending on their instructional goals. In the unit 'air and how things burn', we designed the unit where groups of students engaged in building theories on 'how a candle stops burning in a closed jar'. In the unit 'characteristics of various solutions', groups of students collaboratively constructed a pH scale as knowledge artefact. In both studies, the CSCL technology was implemented mainly for facilitating collaboration between groups. Results showed that: (1) students were more likely to engage in symmetric communication (i.e. between groups as well as within groups) in the second unit, and (2) they were also more idea-centred and more frequently shared their ideas in the second unit. The results were discussed from the perspectives of the scientific practices students engaged in and task structure.