Answer:
What goes up and down, but does not move?
You throw me out when you use me and take me in when you are done. What am I?
What goes up a chimney down but can't go down a chimney up?
What can travel around the world while staying in a corner?
The person who makes me doesn't need me. The person who buys me doesn't use me. The person who uses me doesn't know it. What am I?
No matter how little or how much you use me, you change me every month.