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One, it's impossible to "spot reduce." While you can target certain areas of your body in terms of building up the muscles in that area, you can't decide to just lose weight in your stomach, or your thighs, or your rear. It doesn't work that way. You can't remove subcutaneous body fat from specific areas of the body by doing exercises that target those areas. Doing hundreds of crunches will certainly strengthen your abs, but that won't reduce the amount of fat stored in your torso.
In a way, moderate-intensity physical activity is that "magic pill" a lot of people are looking for, because the health benefits go beyond keeping your waistline trim: Not only can it reduce your risk of cancer, stroke, diabetes and heart attacks, but studies have shown that physical activity can significantly improve the moods of patients with major depressive disorders.