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: You can play me and pull me,
I love the naive.
I'm enjoyable to give,
but bad to receive.

What am I?

Question: Looking at my face, I'm no more than six.
Look at the rest of me and I'm 21.
You'll see my twin and me in Las Vegas.
Say my name and you'd be uttering a threat.

What am I?

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?