Answer:
What can you put in a barrel to make it lighter?
Useful tool for who in darkness dwell. Within you, corrupting like a deadly spell.
What can't you see, hear or feel, until its too late. What shadows love, and shopkeepers hate?
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.
It regulates our daily movements, but it feels no interest in our lives. It directs us when to come and go, but does not care if we pay attention. What is it?
Iām grown from darkness but shine with a pale light. Very round I am and always a lady's delight.