Ambiguous fluidity and rigidity and diamonds that ooze!

If white hemicircles rotate over the edges of a black diamond, there occurs an ambiguity of rigidity and motion. As the hemicircles obscure the vertices of the diamond, the figure transforms from a diamond to a rotating, nonrigid cross made of a tar-like fluid. When the corners reappear, the stimulus again becomes a rigid, solid diamond. Visibility of the vertices implies rigidity. If white squares are rotated, fluidity is not perceived. If the diamond has sawtooth edges and the hemicircles are rotated, no fluidity is perceived. Similarly, if illusory contours suggest the amodal completion of the vertices, rigidity is maintained.