Impossible motion: magnet-like slopes

Winner of the Best Visual Illusion of the Year Contest 2010 Koukichi Sugihara Meiji Institute for Advanced Study of Mathematical Sciences, Japan In this video, wooden balls roll up the slopes just as if they are pulled by a magnet. The behavior of the balls seems impossible, because it is against the gravity. The video is not a computer graphic, but a real scene. What is actually happening is that the orientations of the slopes are perceived op