In this paper we study pupils’ conceptions about fluids and particularly liquids in equilibrium, with reference to the concept of pressure. The study was carried out at two levels. In an in‐depth investigation semi‐structured interviews were administered to Greek compulsory education pupils (13‐14 years) after introductory instruction. Results of the interviews were checked in a survey of pupils’ views by a written questionnaire. Based on pupils’ responses we articulated several mental models which we propose are used by the pupils in order to understand liquids in equilibrium. In the first, the ‘packed crowd’ one, the density of the liquid is considered as non‐constant. In the second, the ‘pressing force’ one, features of pressure and pressing force are confused. In the third, the ‘liquidness’ one, pressure is assigned at a point, rather than exerted on a surface. The implications of the above modelling for introductory instruction about liquids are also discussed.

This essay is a synthesis of more than twenty years of research, already published, on teaching and learning fluids and pressure. We examine teaching fluids globally, i.e., the content to be taught and its transformations, students’ alternative conceptions and their remediation, the sequence of educational activities, being right for students’ understanding, as well as tasks for evaluating their conceptual evolution. Our samples are junior high school students and primary school student-teachers. This long-term study combines research and development concerning teaching and learning fluids and has evolved through iteratively based design application and reflective feedback related to empirical data. The results of our research include several publications.