Silverlight製作右下角伸縮提示

Xaml：
 
      <Canvas VerticalAlignment="Bottom" HorizontalAlignment="Right">
        <Canvas.Resources>
            <Storyboard x:Name="storyPopUp">
                <DoubleAnimation Duration="0:0:2" From="0" To="-200" Storyboard.TargetName="canvasPopup" Storyboard.TargetProperty="(Canvas.Top)" />
            </Storyboard>
            <Storyboard x:Name="storyPopDown">
                <DoubleAnimation Duration="0:0:2" From="-200" To="0" Storyboard.TargetName="canvasPopup" Storyboard.TargetProperty="(Canvas.Top)" />
            </Storyboard>
        </Canvas.Resources>
       
        <Canvas VerticalAlignment="Bottom" HorizontalAlignment="Right" x:Name="canvasPopup" Margin="-300 0 0 0" Width="200" Height="200">                        
            <Border CornerRadius="8 8 0 0" BorderThickness="1" BorderBrush="Red" Background="Azure" HorizontalAlignment="Right" VerticalAlignment="Top"
                     Width="300" Height="200">
                <Border.Effect>
                    <DropShadowEffect />
                </Border.Effect>                    
                <Grid>