What Am I Riddles

Question: There are two sisters: one gives birth to the other and she, in turn, gives birth to the first. Who are the two sisters?

Question: Something all men have,
but all men deny.
Man created it,
but no man can hold it.

What is it?

Question: A hill full a hole full you cannot catch a bowl full.

Question: You wait for it. It cracks, stays for a moment, and then vanishes before your eyes. What is it?

Question: Filled with garb,
the price is free.
Just return what you don't need.

What am I?