This is an example how, depending on the context (i.e., background color), boundaries of objects can be grouped in two different ways (either a fish or a bird). Look closely -- the birds are still there in the water, and the fish are also in the sky, but you don't see it like that, at least not without putting effort. How the visual cortex deals with this bistable figure-ground relationship has been a topic of interest to many vision researchers and most famously was celebrated by M.C. Escher (see the link below).
An editable SVG version of this figure is also provided in the link below.