Good Riddles

Question: I can be one, I can be two. I can be Open, Closed, Empty or Full. I can be Bare, but never a Bear. What AM I??

Question: Say silk ten times. Now answer this question: what does a cow drink?

Question: What is little in Arab and big in Barcelona?

Question: Mike has a father named BOB. Bob is the......... of mike's fatfer

Question: Which is the most curious letter?