Changing Squares
The visitor looks through a hole in a wall into a darkened room. Inside is seen two squares growing and shrinking in size. Since the visitor is constrained to the use of one eye (monocular vision), it is difficult to ascertain whether what is seen is the result of the squares changing size while remaining at a constant distance or changing distance while remaining a constant size (or both). By making a conscious decision, the visitor can see it either way. A button on the wall turns on a light in the room revealing the true nature of the device.
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