The Necker Cube is an ambiguous line drawing. It can be interpreted two different ways. When a person stares at the picture, it will often seem to flip back and forth between the two valid interpretations. The Necker Cube is an optical illusion first published in 1832 by Swiss crystallographer Louis Albert Necker.
Like the paradox of the "half-empty/half-full glass of water" the Necker Cube shows how human perception is multi-stable. It can change and be changed--it can flip/flop.
This also shows why the cvs2bvs brain software is so powerful in the human perception system because it can change, or flip, perception from one stable state to another stable state--on command!