What Am I Riddles

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

Question: What has 6 wheels and flies?

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

What am I?