Logic 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: You have 20 sick sheep, one dies. How many are left?

Question: What does this code mean?: YY U R YY U B I C U R YY 4 ME

Question: Two brothers are twins. One twin says he is older than the other. The other twin is confused and says: in order for us to be twins we have to born on the same day. How is the one twin older than the other?