how to come into being XML format as the response of a .aspx file?

One of the things that asp.net can do is the .aspx file runs at the server and the results(response) in HTML(or XHTML) format is sent to and rendered at the client.

 Now the things I hope to do are: .aspx files run at the server but the response in .xml format as the results and to be sent to and rendered at the client.

Thanks for any help!
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