This paper asks and discusses the general question: what is educational innovation? We examine a ten-year case study investigating a long-term relationship between an elementary school and a university in Sweden. The relationship is jointly constructed and mediated by local, national and international projects. We consider these jointly constructed and shared projects as innovations for both the school’s and the university’s pedagogical practices. Significant actors in the different projects have been teachers, undergraduate and graduate students, pupils, and researchers. The collaboration – which still exists— started in 1996 and has gone through stages that vary in intensity and scope. This relationship may be metaphorically described as a thin string— occasionally almost invisible, yet strong enough to sustain long-lasting collaboration aimed at educational innovation. A historical analysis of the trajectory of the collaborative process will be presented. The analysis is aimed at explaining why this collaboration exists and what makes it sustain itself. The data consist of interviews, e-mail messages, meeting minutes, field notes, project plans, project reports, syllabi, and student reports. The study contributes to our understanding of sustainable educational innovation and to the development of research methodologies for the study of long-term collaborative intervention efforts.