Answer:
Double my number, I'm less than a score. Half of my number is less than four. Add one to my double when bakers are near. Days of the week are still greater, I fear.
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.
My teeth are sharp, my back is straight, to cut things up it is my fate.
I am the red tongue of the earth, that buries cities.
It cannot be seen, cannot be felt, Cannot be heard, cannot be smelt. It lies behind stars and under hills And empty holes it fills. It comes first and follows after, Ends life, kills laughter.
I am your mother's brother's only brother in law. Who am I?