The ID of an HTML Text Control with Runat="Server" Is Being Restated when there is a Master Page

Hi,
<input type="text" id="Test" name="Test" size="15" style="width: 730px" Runat="Server" />
When the page is loaded and there is no Master page the id remains a Test.
When a Master Page is attached it gets renamed as ctl00_ContentPlaceHolder1_Test.
Why is this?
Thanks,
Will
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