I am light as a feather but no one can hold me forever.

Who am I?

If you know me, then you want to share me but if you share me, I'll be gone.

Who am I?

I am under your feet, in the midday sun, you cannot lose me, no matter how much you run.

Who am I?

When you do not know who I am, then I am something. If you do know what I am, then I am nothing.

Who am I?

I am...a riddle.