Answer:
Thousands lay up gold within this house. But no man made it. Spears past counting guard this house, but no man wards it.
A little house full of meat, no door to go in and eat.
What is it which builds things up? Lays mountains low? Dries up lakes, and makes things grow? Cares not a whim about your passing? And is like few other things, because it is everlasting?
A dagger thrust at my own heart, dictates the way I'm swayed. Left I stand, and right I yield, to the twisting of the blade.
What can be heard and caught but never seen?
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?