In this video, Nikolai Tillmann walks us through pexforfun.com, a website where you can write small C#, VB, or F# programs and explore them directly from your browser by using Pex. Additionally, Pex also makes it possible to play coding duels, in which users compete to discover a secret puzzle implementation.   Tune in to watch as Nikolai discusses all of this and plays a duel with Aaron Shaver, our high school intern who has been writing puzzles all summer long. The Research in Software Engineering team (RiSE) coordinates Microsoft’s research in Software Engineering in Redmond, USA.