manual

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The explanation is that the brain tends to interpret
reality by turning to past experiences and images it has
already seen.
When the card is the right way up, the pit (the middle part that stands out)
appears in figure 1. If it is turned upside down, the pit appears in figure 2.
All this depends on the fact that the brain is using a fundamental clue to
find out where the pit is.
The clue is shadow.
Because the position of the light source indicates shadow, the brain takes it
for granted that light always comes from above. The shadows in the picture
influence our perception of what we see. Therefore, it seems like the pit has
changed places.
shadow on the part
that is hollowed out
shadow on the par
t
that stands out
side view
side view