What Am I Riddles

Question: In a marble hall as white as milk, Lined with skin as soft as silk, Within a fountain crystal clear, A golden orb does appear, There are no doors to this stronghold, Yet thieves break in and steal the gold. What am I?

Question: What has 6 wheels and flies?

Question: Occurring once in a hundred years and once in thousands. But never in a day or a year or a century.

Question: Having one, means having something to follow. Once you question it, it means having none. People seek it, but some do and some don't find it. What is it?

Question: I can be flipped and broken but I never move. I can be closed, and opened, and sometimes removed. I am sealed by hands. What am I?