This educational science article illustrates education for sustainability in a theme about Earth Hour (energy conservation) in one Swedish preschool. This case study is based on audio recordings of dialogues between children aged five to six years and preschool teachers. It is guided by critical theory, which is also used as a conceptual tool to explore participation, agency and shared meaning-making. The main findings show that children are engaged and active as equal participants in the dialogue, being eager to understand and take part. However, the intended knowledge content fades into the background when other topics are brought up and as educational framing with purposefully framed play imposes regulations on the constructs of the dialogue content. Further consideration needs to be given to how communicative action together with participation and listening, as well as knowledge development and play, can be intertwined.