One way to lose weight is to just stop eating. It sounds simple but in reality you have to know what you are doing to get any long term weight loss benefits from fasting. Lets look at the weakness and strengths of fasting for weight loss.
Like most forms of dieting, fasting can be used to bring about dramatic but temporary reductions of weight. You can go on a long fast, taking nothing but juice or eating grapes or taking some other low calorie meal substitute for several weeks, and while you are on the fast you will certainly lose weight. However, during the fast you may not be able to carry on your normal social interactions. Its not so easy to sit around sipping carrot juice while the rest of your family is eating a normal meal.
Another problem with long fasting is that your body requires vital nutrients (vitamins, minerals, protein) and your favorite juice may not contain them. Long fasting without proper medical supervision can be dangerous.
Finally, a major problem with this kind of fasting is that once you stop fasting and resume eating your normal diet, you are likely to gain back all the weight that you have lost during your crash weight loss...