retain or copy for UIWebView??

It doesn't work if I give a "copy" attribute to a property of a UIWebView object, such as @property (nonatomic, copy) UIWebView *webv. Then I tried to change copy to retain and my code works fine which can load URL to the view. Wny UIWebView object need retain instead of copy? The same thing happen to UITextField as well. Please let me know.Thanks.