Optical illusions attract young people because they are surprising and mysterious. They can also be used as a starting point for a mathematical topic such as parametrization of planar curves. From a mathematical point of view, this paper describes the path of fixed points on a circle while whirling inside a larger circle. A surprising linear path is evoked under specific conditions and is analyzed by mathematical parameterization, which can be used to start discussing parameterizations of planar curves in a calculus course at university first degree level. Moreover, we provide Maple-code to foster student exploration of paths on whirling inner circles. A description of the implementation and students’ reactions in our own calculus classes is added.
Keywords: parameter; modeling; geometry.