Educational activities using microblogging co-located with face-to-face communication might promote productive classroom interactions. However, much depends on how teachers design those activities. This article explores how the educational design of an activity that uses microblogging engages lower secondary school students in classroom interactions that are productive for learning. It presents a study of one teacher's educational design in which students (aged 12-13) in a Norwegian classroom use microblogging to explore distinctions between facts and opinions. Moreover, the authors consider how the students pick up on the educational design. The findings show that an educational design involving microblogging can provide new possibilities to facilitate peer interactions by systematically enabling students to access more of their peers' ideas, produce and discuss collective ideas and participate in exploratory talk. In particular, the use of hashtags proves suitable for facilitating peer interactions with the aim to develop students' critical thinking.