Moods, say the experts, are feelings that are likely to become fixed, having effects on one′ s out-look ( way of looking at things ) for hours, days or even weeks. That′ s great if your mood is a pleasant one, but a problem if you are sad, anxious, angry or simply lonely. Perhaps the best way to deal with such moods is to talk them out; sometimes, though, there is no one to listen. Modern pharmacology (药物学) offers a lot tranquilizers ( 镇定剂 ) and anti-anxiety drugs. What many people don′ t realize, however, is that scientists have discovered the effectiveness of several non-drug methods to make you free from an unwanted mood. These can be just as useful as drugs, and have the added benefit of being nonpoisonous. So next time you feel out of sorts, don′ t head for the drug store--try the following method. Of all the mood-changing self-help techniques, aerobic exercise seems to be the best cure for a bad mood. "If you could keep the exercise, you′ d be in high spirits, "says Kathryn Lance, author of Running for Health and Beauty. Researchers have explained biochemical and various other changes that make exercise compare favorably to drugs as mood-raiser. Physical work such as housework, however, does little. The key is aerobic exercise--running, cycling, walking, swimming, or other repetitive and sustained activities that increase the heart rate and circulation (循环), and improve the body′ s use of oxygen. Do them for at least 20 minutes a time, three to five times a week. What is the main subject of the passage?