<p>The Rationality Index of Plant Use (RIPU) heuristic was developed as a tool to guide ethnobotanists on the rationality of plant uses by cultural groups. However, this heuristic also holds affordances in science education, to promote self-directed learning, and to provide students with more nuanced understandings of the tenets of science, the nature of indigenous knowledge, and what constitutes pseudoscience. In this paper, the authors reflect on a sequential embedded mixed-method study, where pre-service teachers in a natural sciences course, studying thermodynamics, applied the RIPU heuristic to investigate various commercial health products claiming to support weight loss. The learning activity was structured around problem-based and cooperative learning principles, and 36 pre-service teachers participated in the study. Responses to the Self-Directed Learning Instrument, administered in a pre- and post-intervention context, constitutes the quantitative data. Qualitative data includes the completed RIPU instruments, the presentations that cooperative groups made to their peers, focus group interviews that were conducted with the pre-service teachers, and their reflections on the learning activity. Six major themes were distilled from the data, namely, (a) the RIPU addresses the affective domain, as it fosters enjoyment in the learning process, (b) it is cognitively demanding, and could lead to meaningful learning, development of analytical skills and scientific understanding, (c) following a cooperative learning strategy with the RIPU heuristic leads to critical engagement and the participation of all members, (d) the RIPU provides pre-service teachers with more nuanced understandings of the nature of the natural sciences, and the subject’s syntactical nature, (e) the real-world context makes it very suitable for the classroom, and (f) the RIPU heuristic holds affordances to enhance self-directed learning.</p>