Good Riddles

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 is a word made up of 4 letters, yet is also made up of 3. Sometimes is written with 9 letters, and then with 4. Rarely consists of 6, and never is written with 5.

Question: A man orders a coffee and finds a fly in it. He sends it back and gets another one. But he quickly discovers that they gave him the same coffee that had the fly in it.

How did he know?

Question: Although I'm far from the point, I'm not a mistake.
I fix yours.

What am I?

Question: If you randomly choose one of the following answers to this question, what is your chance of getting it right?

A) 50%
B) 25%
C) 0%
D) 25%