Answer:
Only two backbones and thousands of ribs.
Whoever makes it, tells it not. Whoever takes it, knows it not. Whoever knows it, wants it not. What is it?
Jack and Jill when up the hill jack fell down
What has 13 hearts but none beat?
Talks like this, a green wise man does. Guess my clue, you will.
You use it between your head and your toes, the more it works the thinner it grows.