What Element Within a StackPanel Was Clicked on?

 Is there a way to determine what element in a stackpanel was clicked on?  Like an index of a child or something?  For example:
        <StackPanel x:Name="Menu" MouseLeftButtonDown="StackPanel_MouseLeftButtonDown" HorizontalAlignment="Left" Margin="10,0,0,0" Orientation="Horizontal">
            <Border BorderBrush="#FF014e92" BorderThickness="1,0,0,0">
                <Rectangle x:Name="button1" Fill="#FF4c87b3" Width="100" Height="100" />
                </Border>
            <Border BorderBrush="#FF014e92" ...