The Ames Window - A Bizarre Illusion
The Ames Window is an optical illusion, in which a strangely-shaped window (with a stick running through it) is rotated slowly. Because we assume it to be a normally-shaped window, our brains perceive the window and the stick interacting in strange ways, seeming to change shape in ways that don't align with normal perspective shifts. The illusion was designed by Adelbert Ames, Jr., an American artist and psychologist. Ames also created the Ames Room, a similar perspective illusion that was later used to film scenes in The Lord of the Rings, making hobbits appear small. Neat stuff. Here's a bit more from Wikipedia on the Ames Window, followed by an explanatory video:
The Ames trapezoid or Ames window is a style of window which, when observed frontally, appears to be a rectangular window but is, in fact, a trapezoid. The window is mounted on a rod connected to an electric motor that rotates it about its vertical axis. When it is observed with one eye from about 3 meters or with both eyes at 6 meters, or more, the window appears to rotate through 180 degrees and then seems to stop momentarily and reverse its direction of rotation. It is therefore not perceived vertically to be rotating continuously in one direction but instead is mis-perceived to be oscillating, reversing its direction once every 180 degrees.
This video shows the window, with commentary by Professor Richard Gregory.