Within the field of technology education, evaluating technical solutions is considered as an important topic. Research indicates that pupils have difficulties in evaluating technical solutions in terms of fitness for purpose, i.e. how effective a technical solution supports its intended function. By using the learning study, which is an iterative and collaborative research approach, the study explores the capability to evaluate technical solutions in terms of fitness for purpose, what it takes to know it and how to best enhance its learning in the primary technology classroom. Audio and video recorded interviews, teachers' meetings and lessons are the base data for the study. A contribution of the study is the understanding of this specific knowing, and what is critical for learning and thus to an improvement of technology teaching practice.