When you don't know what I am, I am something

Question: When you do not know what I am, then I am something. But when you know what I am, then I am nothing. What am I?

Question: What do you call a sad bird?

Question: What has a bed that you can't sleep in?

Question: Where do all the letters sleep?

Question: How are doughnuts and golf alike?

Question: What cannot be seen or felt, but can only be broken by voice?