1) Are you kidding=Are you pulling my leg??

　　你在开玩笑吗?

　　For 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!

　　For example:

　　--Andy, you didn't pass the exam!

　　--I know you are kiddding.

　　--No kidding.