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: Why did the hen cross the road?

Question: What do you call a sad bird?

Question: What is an astronaut's favorite key on the keyboard?

Question: What is the tallest building in the world?

Question: What tastes better than it smells?