It is possible to lose a pound of weight in a day, but it is not possible to lose a pound of fat a day! As I already discussed in my page on how to lose one pound a week, you need to spend extra 3500 calories to lose one pound of fat. Now if your basic metabolic rate is 2000 calories, you have to spend 5500 calories a day. Which means you have to walk 70 miles on that day to lose one pound of fat on that day. It is not practical to walk 70 miles a day. But if you take some sorts of fad diets, like some sorts of tea, or other medicines, you may lose lot of water first few days. It is very much possible to lose one pound of body fluid on the first day. So if you check your weight you may find you lose one pound on that day, but actually your weight shows less because you lose water not fat.