While there are many fitness apps, this Opportunity chapter asks you to step back and think about how physical activity and sport inform UX design. How do current apps accommodate being mobile rather than just being on a mobile? What about “hard to use” as a design principle, the effort being like a difficult run, the reward like the post race satisfaction? In some activities such as cycling or horse riding, the person and “tool”—a bike or horse—can feel as one. We contrast this “centaur” design to the predominant “cyborg” design in mobile UX.