Only a few games seem to have successfully achieved that balance. Some were simply too hard, others too easy, and those aren’t really the only variables anyhow. But the best could sustain excitement, or even induce a “flow state,” that form of contentment, of “optimal experience,” described by the cognitive scientist Mihalyi Csikszentmihalyi, in which people feel “strong, alert, in effortless control, unselfconscious, and at the peak of their abilities.” It was more than enough to keep people coming back, in their billions, across the world, and parting with their hard-earned money for a chance at such transcendence.

Tim Wu, The Attention Merchants, loc. 684