Answer:
Cloud is my mother, wind is my father. What am I?
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.
What force and strength cannot get through, I with my unique teeth can do. What am I?
I help you carry your books.
Never ahead, ever behind, yet flying swiftly past, for a child, I last forever, for adults, I'm gone too fast.