Answer:
I'm the source of all emotion, but I'm caged in a white prison.
Three little letters, a paradox to some. The worse that it is, the better it becomes.
Runs smoother than any rhyme, loves to fall but cannot climb.
Sleeping during the day, I hide away. Watchful through the night, I open at dawn's light. But only for the briefest time, do I shine. And then I hide away. And sleep through the day.
Useful tool for who in darkness dwell. Within you, corrupting like a deadly spell.
What is so fragile that saying its name breaks it?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?