Good Riddles

Question: Like a middle pillar I stand among 6, all 9 of my characters make me the largest to exist.

Question: Keys! keys! So many keys, When F is involved The key note is _ ?

Question: When people cut me, I do not bleed. They use me, but then they leave. I die, but then revive. I change my appearance as time goes by. What am I?

Question: At first I'm in pieces, but you build me and I carry you, at night when your tired or sleepy or even drunk, what am I?

Question: Although I am important to you, I am but a mere illusion. My beginning is as unknown as my conclusion. The more that you think, the more that I appear to pass. When you are in pleasure, I tend to escape your grasp. What am I?