Good Riddles

Question: Without me, nothing exists. What am I?

Question: Until I am measured, I am not known. Yet how you miss me, When I have flown What am I?

Question: I am very strong and tough but can be broken in some ways. What am I?

Question: What sound do porcupines make when they kiss?

Question: What occurs once in a minute, twice in a moment and never in one thousand years?