In the stock market (as in much of life), the beginning of wisdom is admitting your ignorance. One of the many things you cannot know about stocks is exactly when they will up or go down. Over the long term, stocks generally rise at a nice pace. History shows they double in value every seven years or so. But in the short term, stocks are just plain wild. Over periods of days, weeks and months, no one has any idea what they will do. Still, nearly all investors think they are smart enough to divine such short-term movements. This hubris frequently gets them into trouble.