Answer:
My teeth are sharp, my back is straight, to cut things up it is my fate.
This is needed both for courage and hardcover books.
What runs around all day. Then lies under the bed. With its tongue hanging out?
I'm sometimes white and always wrong. I can break a heart and hurt the strong. I can build love or tear it down. I can make a smile or bring a frown.
An open ended barrel, it is shaped like a hive. It is filled with the flesh, and the flesh is alive.
Before I grow I'm small. When I'm old I grow tall. When I die I give a mighty fall.