The surface in above image is a portion of the graph of
some mathematical equation.
Let's do some brainstorming:
Can you guess what's inside the surface?
Consider the intersection of a horizontal plane with the surface.
The intersection consists of a family of funny curves.
(An example curve is shown below.)
Cut off the upper surface above the horizontal plane
to get a funny 3D image.
(This curve is obtained from another surface and has nothing to do with the answer.)