What Am I Riddles

Question:

The dirtier I am, the whiter I get,
Leave a mark on me when you stand, I'll leave a mark on you when you sit.

What am I?

Question: I am the only organ that named myself.What am I?

Question: I have two rings. I move by request. If the mixture isn't right I just sit and rest.

What am I?

Question: What is as big as you are and yet does not weigh anything?

Question: Without me, nothing exists. What am I?