Answer:
To unravel me you need a simple key, no key that was made by locksmith's hand. But a key that only I will understand.
Weight in my belly, Trees on my back, I travel fast in my place, but feet is a feature I lack
I dig out tiny caves and store gold and silver in them. I also build bridges of silver and make crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs my help. Yet many people are afraid to let me help them.
Twigs and spheres and poles and plates. Join and bind to reason make.
As round as an apple. As deep as a cup. All the king's horses can't pull it up.
What did the piece of wood say when he saw the screwdriver and screws approaching?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?