Good Riddles

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: When is 99 more than 100?

Question: When you stand up he sleeps When you sleep he stands Who is it?

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: Can you rearrange the letters in new door to make one word?