<Canvas xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Rectangle x:Name="rect-one" Fill="Red" Stroke="Black" StrokeThickness="3" Canvas.Left="10" Canvas.Top="10" Width="100" Height="100">
        <!-- it's invalid (i.e. NOT working in SL) to have X.Triggers to define events inside Rectangle -->
	<X.Triggers>
	    <TriggerCollection>
	            <EventTrigger RoutedEvent="Rectangle.Loaded">
	                <BeginStoryboard>
		            <Storyboard>
		                <DoubleAnimation
			            Storyboard.TargetName="rect-one"
			            Storyboard.TargetProperty="Width"
			            From="10" To="300" Duration="0:0:30"
			        />
		            </Storyboard>
		        </BeginStoryboard>
      	            </EventTrigger>
	    </TriggerCollection>
	</X.Triggers>
    </Rectangle>
</Canvas>
