Good Riddles

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

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: What is AD before BC, exams before tests and last year is after last month?

Question: what does not move outside (the surface)but moves inside ?

Question: what have holes but you will see it on the street ?