Flow-It should be part of every good video game.

One evening you sit down to play a game and you become so involved, you lose all track of time, you are finding and defeating the bad guys effortlessly, you are easily finding the clues and you are engaged in the challenge. As you play, you are nervous but confident that you can move forward and achieve the goal of this level. Nothing is going to stop you. Four hours later you suddenly realize you are hungry. You played right through dinner time and didn’t get anything to eat—not even a snack.

If you’ve ever had that experience playing a game, working on a hobby, riding a bicycle or completing a task at work you’ve experienced what Mihaly Csikszentmihalyi calls “Flow.”
Flow is a mental state of operation in which a person is fully immersed and focused in what he or she is doing; it involves full mental involvement and continual engagement in the process of the activity.

Flow is elusive and cannot always or easily be designed into a game. It is even harder to test for flow because it is hard for a designer of a game to get into a flow state with their own game. But flow is something game designers want their players to achieve. And while a game designer cannot guarantee a flow state will occur in the player, the designer can create conditions under which a flow state could occur.

Flow is experienced when the challenge facing a person is in almost perfect balance with the person’s level of skill and abilities—they can accomplish the task but it will take concentration, a blocking out of distractions and a high level of effort.

What flow "looks" like.