Answer:
In marble halls as white as milk, lined with a skin as soft as silk. Within a fountain crystal-clear. A golden apple doth appear. No doors there are to this stronghold, yet thieves break in and steal the gold.
I am the heart that does not beat. If cut, I bleed without blood. I can fly, but have no wings. I can float, but have no fins. I can sing, but have no mouth.
It flies when it's on and floats coming off.
Twigs and spheres and poles and plates. Join and bind to reason make.
When I get closer my tail grows longer, but when I go away my tail leads the way.
What never gets any wetter no matter how hard it rains?