In this episode, Sundara Kumar and Mark Harper work on the new custom markup extensions (CME) feature in Silverlight 5. They explain how CME's allow you to extend XAML so that you can run any custom code. These guys (along with some other members of the team) literally wrote the feature. Mark was also instrumental in writing the CME that I used in many Silverlight 5 demos to hook events up to a method in a ViewModel.