Vista - Changing the video associated with a MediaElement object?

Hi,

I can't figure out how to change the video associated with a MediaElement
object at runtime. I was hoping to do it in the C# code behind.

With Expression Blend I can set the original video associated with the
MediaElement object, and it plays just fine. On the same window, I have
some buttons that I'd like to use to change which video is playing. But I
can't see any way to change the video associated with the object? I tried
setting the "Source" property, in the C# code behind, but the property is
readonly.

Do I need to do it in XAML rather than C#?

Any help would be appreciated.

Thanks,
Dan