Rendering a stiff virtual surface using a force-feedback haptic interface has been one of the most classic and important research issues in haptics. In this research, we present a novel haptic rendering technique, named stiffness shifting, which greatly increases the perceived hardness of a virtual surface. The key idea of stiffness shifting is to use a stiffness profile that includes an instantaneous increment shortly after a contact. The algorithm is very simple, and can be easily integrated into existing haptic rendering algorithms for 3D objects.