boredom, as it does not fully engage the attention, while a too difficult one produces anxiety, as theperson becomes afraid to fail. Only a task that is challenging enough will engender the level ofintense, but tranquil, concentration that characterizes flow. There are two ways to control thebalance between challenges and skills: changing the intrinsic difficulty of the task, and changing theperson’s ability to cope with the task. At first sight, balance could be achieved by proposing arelatively easy task at which the person is not particularly skilled. But a more advanced model seesflow as emerging from high skills applied to difficult challenges (Fig. 1). In this more complexmodel (Nakamura & Csikszentmihalyi, 2002), limited skills applied to limited challenges merelyproduce apathy, as there is not much to create interest.This means that a good mobilization system not only should present goals that are difficultto reach, but provide the additional abilities necessary to handle that difficulty. This is the “newskills” feature that characterizes a truly compelling technology: you will feel most stimulated to usea tool if it allows you to tackle challenges that you could not tackle without it—albeit in such a waythat its use is fully intuitive and transparent. Eventually, a good tool should start to feel like anaugmentation or extension of yourself—the way a stick extends the reach of your arm, a telescopeextends your vision, and a notebook extends your memory.