Science pedagogical content knowledge is the special amalgam of content and pedagogy that is uniquely the province of teachers and is a key focus in science teacher education. Primary school teachers and junior high school teachers who teach outside of their main area of expertise often have limited science content knowledge which is manifest as limited science PCK. Some primary school teachers use activities that work and while junior secondary science teachers use units that work as a supplement to, or substitute for, science PCK. This study explores the extent to which activities and units that work played a role in primary and secondary teachers’ planning and implementation of a new outcomes-based science syllabus. Using two case studies, we show that activities and units that work played a major role in the teachers’ thinking and teaching. The primary school teachers relied on activities from a number of sources, whereas the high school teachers tended to rely on units from their textbook. This study has implications for teacher professional development and pre-service teacher preparation.