how to resize border slowly slowly not suddenly

Hi,

I have following code for border animation.

MainPage.Xaml :

Here DoubleAnimation Storyboard.TargetName="bTest",bTest is the name of Border.

Mainpage.Xaml.cs :

 public MainPage()
        {
            InitializeComponent();
            this.Loaded += new RoutedEventHandler(MainPage_Loaded);
        }
//Start animation on load event of page
 void MainPage_Loaded(object sender, RoutedEventArgs e)
        {
           //borsoryboard is the name StoryBoard which is used for animating the border.

            this.borsoryboard.Begin();
        }

Thanks.