Answer:
I am a desire to munch
I regularly change shape yet remain a sphere. I am always there but not often seen. What am I?
You can see it everyday, but cannot touch it at will. What is it?
The more you work the more I'll eat.You keep me full I'll keep you neat.
A single syllable do I claim black was my most famous name; Fetal to mortals here below thousands have I slain in a single blow.
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?