The authors investigated how prospective teachers enrolled in an undergraduate physical science course participated in an online forum in which they posted reactions to video episodes of children talking about science. Using Positioning Theory (Harre & Van Langenhove, 1991) as a lens, the authors analyzed 108 online posts from 26 prospective teachers as they completed six prompts from a Unit Task about force. Prospective teachers compared their own current ideas about physics topics to their prior understandings as well as to ideas articulated by the children in the video clips. Additionally, within these posts the prospective teachers positioned themselves as knowledgeable about how physics ideas develop, an important aspect of teaching science. As the prospective teachers wrote about the videos in their online posts, the videos may have served as a point of comparison with which they could document their understanding of physics concepts as well as the process of learning physics.