Answer:
What is given but kept by the giver?
You use it between your head and your toes, the more it works the thinner it grows.
Three little letters, a paradox to some. The worse that it is, the better it becomes.
Only two backbones and thousands of ribs.
I am the greatest teacher out of them all, but alas in the end I kill my pupils.
I have two eyes in the front and a lot of eyes on my tail
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?