The aim of this study was to develop a new research-based learning unit for tides to be used in lower secondary schools. The learning unit was based on the scientific theory of tides, textbooks, and also an analysis of students’ conceptions. Descriptions are included of the content and the teaching–learning activities of the unit. The teacher talk was analysed to discover possible information about the reasons underlying the learning effects observed. A pre-test revealed that students in the research groups had the same kind of misconceptions that we had found previously in other student groups. One month after the teaching period, the test showed that the students had learned about tides to a considerable degree of proficiency. The reasoning displayed by the students in the course of receiving the unit of teaching also revealed the eventual learning outcomes. Discussion is also presented of the implications of the project for the planning and implementation of research-based teaching sessions.