Ames Window

The Ames window, also known as Ames trapezoid, is a trapezoidal shaped window that, when viewed frontally, looks exactly like a normal rectangular window viewed from an angle. The original experiment consists of this window, mounted on a metallic vertical pole, slowly rotating around its vertical axis.

When viewed with one eye from a distance of 3 meters, or with two eyes from a distance of 6 meters or more, the observer sees the window rotate for 180 degrees, then stop briefly, then reverse its direction of rotation (even though the window is effectively not inverting the rotation). This effect is made even more remarkable when a stick is placed inside the window frame, positioned perpendicularly to the window: the observer usually sees the stick "pass through" pieces of the frame at each 180 degrees rotation, even though it's evidently impossible.