What Am I Riddles

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: After you go through a fall I will take over.All life will stall, or at least grow slower.What am I?

Question: Countless blades that bend with a touch,
exploited by kids who want to make a buck.

What am I?

Question: I am a big threat to humanity,
but you wouldn't get rid of me even if you could.

What am I?