如何获得弹出动画

这是我的代码（假设我们在页面上有一个按钮来启动弹出窗口）：

    private void button1_Click(object sender, RoutedEventArgs e)
    {            
        Rectangle rect = new Rectangle();
        rect.Width = 50;
        rect.Height = 50;
        Popup mypop = new Popup();
        mypop.Child = rect; // just to put something inside the popup for now           
        mypop.PopupAnimation = PopupAnimation.Slide;
        mypop.StaysOpen = false;
        mypop.PlacementTarget = button1; // open next to the button
        mypop.IsOpen = true;
    }

弹出窗口只是在没有设置动画的情况下出现，即使我设置了PopupAnimation属性。。有人知道为什么吗？