Flow theory, put forth by Csikszentmihalyi suggests that the greatest level of engagement comes when we are challenged at the peak of our ability, but no more. As applied to games, the level of challenge should increase proportionally with the player's level of ability. If the challenge increases faster than ability, the player will become frustrated when he or she is no longer able to make progress. If the challenge increases slower than ability, the player will become bored. Flow is motivation for why games should intelligently adapt their difficulty.