How the IIS server knows in Asp.net that what all control it has to load?

Every ASP.NET server control is identified as a sever control by the "runat" attribute this attribute is set to server like this
<asp:Button id="Mybutton" runat="server" this will identify all server controls..

In any case all controls on a page are added to the controls collection of the page.

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BrainBench ADO.NET and ASP.NET Certified Developer