Via MindHacks:

Perception is a fundamentally underconstrained problem. You get information in through your senses, but not enough information to be absolutely sure of what is causing those sensations. A good example is perception of depth in vision. You get a pattern of light falling on your retinas (retinae?), in two dimensions, and from that you infer a three dimensional world, using various clever calculations of the visual system and some assumptions about what is likely. But because the process remains fundamentally underconstrained, there is always the possibility that you will see something that isn't really there - that is, your visual system will take in a pattern of information and decide that it is more likely to be produced by a scenario different from the real one.

They're painted so that from one particular angle the shapes line up and your visual system flips into thinking that it can see a flat, 2D, pattern when the reality is a disjoint 3D one.

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