1） Are you kidding=Are you pulling my leg??　　你在开玩笑吗?

　　example:

　　-do you know that Andy is going out with someone?
　　-what? are you kidding? i am still a bachelor.
　　-so who is that girl walking along the street with you yesterday?
　　-my god! she is my sister and she has got married and her baby is 5 years old!
　　-wow, your sister looks so young.

　　2） No kidding!

　　我不是在开玩笑.=I am serious!

　　example:

　　-Andy, you didn't pass the exam!
　　-I know you are kiddding.
　　-No kidding.