Three spheres comprised the world. The waters, the earth, and the skies. Upon first glance, it would seem that every creature lived in its own sphere. The creatures of the waters lived in the waters. The creatures of the earth lived upon or beneath the earth. But the world was not so sharply divided. And one of the spheres was not truly inhabited by any creatures.
The creatures of the skies did not live in the skies. They soared and fluttered, sometimes for long stretches of time, but a time would come when they need descend and perch.
Some creatures lived where these spheres meet. Some creatures lived part of their lives in one sphere and part in another.
Once, in a long-forgotten age, every creature could live in every sphere.
This was so because of an energy, a force that spun around and through the world.