Answer:
Whilst I was engaged in sitting. I spied the dead carrying the living
Only two backbones and thousands of ribs.
My second is performed by my first, and it is thought a thief by the marks of my whole might be caught.
Neither bought nor sold but more valuable than gold. It is built but not by hand. What is it?
Flat as a leaf, round as a ring. Has two eyes, can't see a thing.
You use it between your head and your toes, the more it works the thinner it grows.
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?