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: Where do all the letters sleep?

Question: What tastes better than it smells?

Question: What goes in the water black and comes out red?

Question: What loses its head in the morning and gets it back at night?