[Corpora-List] Looking for some corpora about why-questions, how-questions, and their answers.

[tzshen]

Dear all,

I am doing some work to find the answer patterns 
to help automatic answering some complex questions, which ask for a complex answer.
I first focus on why-questions and how-questions.
So I am eager to find some corpora that contains large amount of this two types of questions and corresponding answers.
Does anyone know where I can find this kind of corpora or related resources?
Resouces about other complex questions and answers beyond why- and how-questions are also welcome.

THANK YOU ALL VERY MUCH.

Tian-Zuo, Shen