The transformation of a solid to its net is based on something quite different from simple perceptual impression. It is a mental operation performed by manipulating mental images. The aim of this study was to observe pre-service and in-service teachers' ability to visualize the transformation of a curved solid to its net and vice versa, and to try to classify and to analyze students' mistakes and difficulties related to this kind of visualization. The most significant conclusion is that when lacking any experience, many students are unable to imagine the process of unfolding a curved surface. The findings show poor performance in attempts to produce nets or judge drawings as being possible nets of a cylinder or a cone. (Contains 7 figures and 2 tables.) [For complete proceedings, see ED500859.]