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: If you have a cake, how many pieces of cake can you form with 3 straight cuts?