A "user interface" is the part of an interactive system that bridges the user and the underlying functionality of the system. But people sometimes forget that the best interfaces will provide a platform to optimize the users' interactions so that they support and extend the users' activities in effective, useful, and usable ways. To look at it another way, users and systems are bound in an infinite loop of questions and answers. Ineffective interactions will occur if an interface does not fully enable the users' questions to be posed, whereas the best interfaces are known to not only be able to support all the questions asked by users but also recommend interactions that extend the user's activity in ways that make his or her journey through the system more effective and satisfying. In this article, the author discusses how to design a user interface by developing personas around the various active users of the products.