Why cant I put a url to another site in the siteMapNode

[Guest]

<siteMap xmlns="http://schemas.microsoft.com/AspNet/SiteMap-File-1.0" >
<siteMapNode url="" title="Menu" description="" roles="*">
<siteMapNode url="~/" title="Home" description="Home">
<siteMapNode url="~/Home.aspx" title="Home" description="Home" />
<siteMapNode url="Todo.aspx" title="To Do" description="To Do" />
<siteMapNode url="http://www.google.com" title="Blah" />

The Home and Todo nodes render in the menu, why doesn't google