What Am I Riddles

Question: You wear me every day but you never put me on.
I will change colors if you leave me out too long.

What am I?

Question: You find me behind the stars; or in a sixth, seventh, or third.
It takes something round, a computer, and me to make pie.
I am bigger than anything you can think of.

What am I?

Question: I'm tall when I'm young and I'm short when I'm old.What am I?

Question: What goes up and down the stairs without moving?

Question: There is an ancient invention still used in some parts of the world today that allows people to see through walls.What is it?