There is no defined time limit for water fasting, however medical counsel normally advises ranging from 24 hours to 3 days as the greatest amount of time to go without eating. Some people can go longer than this, while others must eat after 24 hours even if they are still feeling fine. The body is made up of approximately 60% water. Water is required for so many processes within the body that not drinking enough can lead to serious problems. For example, without water your blood would collapse and organs such as the heart, lungs, and kidneys would fail. When you drink only water for a long period you lose weight because water is needed to maintain muscle mass. Also, water is needed to digest food and absorb nutrients including vitamins and minerals. Some people who fast for religious reasons may stay hydrated by drinking tea or coffee but should avoid alcohol during their fast. The caffeine in these drinks acts as a diuretic-which means it makes you pee more-and since you're not eating anything else with lots of water content like carbohydrates or protein, you need to replace those fluids. Unfortunately, most restaurants don't give out free water bottles, so you'll need to buy your own. There are also water filters available at grocery stores and some convenience stores that will filter out any harmful bacteria or chemicals in the water.