cant animate a rectangle

I draw a rectangle and i cant animate it? it just appears in middle of the screen.

   
        Dim brush As New SolidColorBrush
         brush.Color = Colors.Black

 

        Dim bb As New Rectangle
        bb.Width = 100
        bb.Height = 100
        bb.Fill = brush

        ' Canvas.SetTop(bb, 10)
        '  Canvas.SetLeft(bb, 10)
        LayoutRoot.Children.Add(bb)

        Dim dur As Duration = New Duration(TimeSpan.FromSeconds(0.4))


        Dim xa As DoubleAnimation = New DoubleAnimation 'Animation for shrink Width
        Dim ya As DoubleAnimation = New DoubleAnimation ' ...