According to Escher: “In the horizontal center strip there are birds and fish equivalent to each other. We associate flying with sky, and so for each of the black birds the sky in which it is flying is formed by the four white fish which encircle it. Similarly swimming makes us think of water, and therefore the four black birds that surround a fish become the water in which it swims.” The birds take on an increasing three-dimensionality in the upward direction, and the fish, in the downward direction. But as the fish progress upward and the birds downward they gradually lose their shapes to become a uniform background of sky and water, respectively.
This is an example of a figure-ground reversal.
10 Have you not read this scripture passage:
‘The stone that the builders rejected
has become the cornerstone;
11 by the Lord has this been done,
and it is wonderful in our eyes’?”