text: If you eat or drink a pound of anything, even water, you will TEMPORARILY gain one pound.  But long term weight gain is determined by the amount of calories in food, not how much it weighs.  A pound of water has zero calories and will just get pee'ed out.  A pound of spagetti has several hundred calories.  Our bodies burn calories constantly, for most people between 2000 - 3000 per day, more with exercise. 3500 unburned calories is equal to 1 pound of weight gain.  So, if you wanted to gain 1 pound in 1 day, and your body burned 2000 calories per day, you could accomplish this by eating 5500 calories (that is, 3500 extra calories) in that day.