Answer:
I go through a door but never go in and never come out.
I am a creature with a βtightβ name.
I have two body parts but am used to measure length. What am I?
I am used when you jump off a bridge for fun.
It's been around for millions of years but is never more than a month old. What is it?
No matter how little or how much you use me, you change me every month.