<p>This study explores the influence of external resources on students’ construction of mental representations of curved spacetime and their understanding of General Relativity (GR). Using the Cognitive Mediation Networks Theory (CMNT) as the theoretical framework, a short extracurricular course with Year-12 students was developed. Through the course, we investigated how interactions with various external resources support the development of mental representations that facilitate reasoning about relativistic phenomena. Using a case study with qualitative analysis, data from pre/post-tests, interviews, gesture analysis, and student artifacts revealed distinct patterns between students with satisfactory and limited understanding of curved spacetime; students expressed their understanding using multiple representations that reflected their mental representations. Students with accurate conceptions exhibited similar imagistic mental representations associated with the rubber-sheet analogy within psychophysical and hypercultural tools, applying them to explain various situations. Conversely, students with limited curved spacetime conceptions attributed spatial phenomena to forces and associated time dilation with being in ‘outer space’. The findings underscore the importance of carefully selecting external resources, considering students’ prior knowledge, and addressing misconceptions in GR instruction.</p>