Answer:
What has many teeth but can't bite?
What goes through a door but never goes in and never comes out?
The shorter I am the bigger I am.
I have a hundred legs but cannot stand. I have a long neck but no head. I cannot see and I help keep your house neat and tidy.
Iron roof, glass walls, I burn and burn but never fall. What am I?
No matter how little or how much you use me, you change me every month.