Cool Optical Illusion
Just press space. When the shape is moving, you can see it. When it isn't, you can't.
IS YOUR MIND BLOWN YET?
For proof that the square doesn't dissapear, press M before pressing space.
It works because the human brain looks for moving patterns, but there really is no other recognizable pattern in the static. That way, when the square stops moving, there is nothing to separate it from the background and it just dissapears.
The noise background was made with Blender.