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 fish only swims at night?

Question: What do you call a mad elephant?

Question: What kind of mouse does not eat, drink or even walk?

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

Question: Where do all the letters sleep?