What Am I Riddles

Question: Under pressure is the only way I work,
and by myself is the only way I'm hurt.

What am I?

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: The person who buys me doesn't need me, the person who makes me doesn't want me, and the person who uses me can't appreciate me.

What am I?

Question: What is something that you always have with you but you always leave behind?

Question: I have two arms, but fingers I have none. I've got two feet, but I cannot run. I carry well, but I carry best with my feet off the ground. What am I?