A rotating blue square is partially hidden by four red squares. The blue square appears to grow and shrink as it turns.
Shrink the red squares by dragging your mouse across the image. As the red squares get smaller, it is clear that the blue square is not changing size at all. Now drag the other way to make the red squares bigger. The bigger the red squares get, the less we see of the blue square, and the harder it gets to judge its size and shape.
Click the Triangle button to demonstrate the effect with a different shape.