The core mechanism for displaying Silverlight pages is the UserControl class.  The UserControl element appears at the top of every user control.  This root element is a prime choice for storing static resources (discussed briefly later), as well as any namespaces (Silverlight uses namespaces to point to namespaces in an assembly).

The UserControl class is a content control, which means that it has only one property for storing children: Content.  This means only one root level element can appear underneath the UserControl class, excluding complex properties (like UserControl.Resources, where Resources is a collection-based property).

Fortunately, other controls can contain a collection of children, so the user control can host a single control, which that single control can then host multiple controls.  We'll get into the specifics of this soon.  The first control that appears at the top of the user control by default is discussed in the next section.