title: adding one view contoller as a child of another view controller in objective-c
body: <p>I need to know how to add one view contoller as a child of another view controller in objective-c. 
       As of now I am presenting a view controller as a modal of the rootview controller. But I want to change it now. I have 2 view controllers which I am presenting as modals of root view controller. But how can I present them in a different way?  How does this parent-children concept work?  Please give me some references.</p>