Every September, a quiet, churchy city in the American heartland undergoes a Technicolor transformation. Art pops up everywhere—paintings, giant insect statues, experimental happenings. Sidewalks and parks turn into open-air museums; taverns become galleries. A huge pot of money is dangled before the artists, $250,000(!) for the grand prize. And here's the best part: You know who gets to pick the winner? You do.