Answer:
What is often returned, but never borrowed/
By Moon or by Sun, I shall be found. Yet I am undone, if there's no light around
Two brothers we are, great burdens we bear. All day we are bitterly pressed. Yet this I will say, we are full all the day, and empty when go to rest.
I work when I play and play when I work.
I am a box that holds keys without locks, yet they can unlock your soul.
What is so fragile that saying its name breaks it?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?