What Am I Riddles

Question: Tear one off and scratch my head. What once was red is black instead. What am I?

Question: When I come in, someone else leaves, I know everyone, yet no one knows me. I'll meet you all only once in your life, Causing mayhem and mischief and sorrowful strife. What am I?

Question: My thickness is no more than a few centimeters, yet my depth can span to infinity. Although mundane, I intimidate most animals. When in my presence, you will inevitably follow me. What am I?

Question: I can be half without getting thinner.
I can shine with no fire.
I can be hidden but never taken.
I can stay dry while moving the ocean.

What am I?

Question: I have 6 faces and 21 eyes, but can't see a thing. What am I?