This qualitative study investigates how young students expressed, explored and expanded their ideas of how they could contribute to sustainable development. We analyse a trajectory in a Norwegian 8th grade class, focusing on the students' emerging understanding through microblogging and talk in individual, group and whole class activities. The material analysed are logs from a microblogging tool and transcribed video recordings from two lessons. We examined the trajectory as a whole to understand how the topic was treated, we sorted students' microblogs thematically, and we used interaction analysis to investigate talk excerpts. The results show that the students initially tended to suggest everyday actions related to reducing consumption, and that through classroom interactions about a challenging idea their understanding broadened. The combination of microblogging activities and discussions facilitated the travelling of ideas between activities and participants, prompted the students to question, elaborate and reason, supported broad participation, and helped students create links between everyday actions and wider sustainability issues.