Use of self-generated analogies has been proposed as a method for students to learn about a new subject by reference to what they previously know, in line with a constructivist perspective on learning and a resource perspective on conceptual change. We report on a group exercise on using completion problems in combination with self-generated analogies to make sense of two thermodynamic processes. The participants (N = 8) were preservice physics teacher students at the fourth year of the teacher education program. The students experienced challenges in accounting for the constant entropy in reversible, adiabatic expansion of an ideal gas and the constant temperature in free, adiabatic expansion of an ideal gas. These challenges were found to be grounded in the students' intuitive understanding of the phenomena. In order to come to terms with the constant entropy in the first process, the students developed idiosyncratic explanations, but these could by properly adjusted given suitable scaffolding. In contrast, the students by themselves managed to make sense of the constant temperature in free expansion, by use of microscopic explanatory models. As a conclusion, self-generated analogies were found to provide a useful approach to identifying challenges to understanding among students, but also for the students to come to terms with these challenges. The results are discussed against a background of different perspectives on the issue of conceptual change in science education.