Turn one photo over and they look the same: two holes, or two hills.
Faces everywhere
Crater illusion
Is each photo a hole or a hill?
Most people see one of each, a hole and a hill, though it’s the same photo upside down. (It’s a crater on the Moon.)
Why? Your brain assumes light comes from above, like the Sun, and the shadows only make sense one way: as a hole, or as a hill. Turn the photo over and the light seems to come from the other side, so it flips.
Picture: NASA/GSFC/ARIZONA STATE UNIVERSITY, Public domain · Wikipedia(opens in a new tab)