In Blend you use the Pen tool to draw lines and they appear in the XAML as <Path> elements. But after you’ve drawn your lines and you test your application, the Paths appear immediately: they don’t replay the gestures you used to create...(
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Read the complete post at http://blogs.msdn.com/expression/archive/2008/08/15/how-to-animate-a-drawing-line.aspx