More than 3,000 haunted theme attractions open their doors nationwide every Halloween, earning an estimated $500 million. It turns out there's a science to creating a good scare. And the most effective attractions are designed to slowly build a feeling of controllable fear and anxiety. "There are safe thrills," said Frank Farley, a psychologist at Temple. "It's not like climbing Mt. Everest. You know you are going to come out O.K."