Answer:
What month of the year has 28 days?
My life can be measured in hours. I serve by being devoured. Thin, I am quick. Fat, I am slow. Wind is my foe.
What tree that definitely grows does not have a shadow and does not grow fruit?
Die without me, never thank me. Walk right through me, never feel me. Always watching, never speaking. Always lurking, never seen.
What I am filled, I can point the way. When I am empty. Nothing moves me. I have two skins. One without and one within.
I'm the source of all emotion but I'm caged in a white prison.