Shapes that aren’t there
Illusory sphere
Do you see a white ball in the middle, with spikes sticking out of it?
There is no ball: no edge is drawn there, only the cut-off ends of the spikes.
Why? Each spike ends in a curve, as if it came out of a round surface, and they point different ways, some toward you. So your brain builds a whole ball out of the ends, in 3D, and even sees its edge.
Picture: Vectorization: Mrmw, CC BY-SA 4.0 · Wikipedia(opens in a new tab)