Take a person that with X% body fat and Y% muscles. They start going to the gym; after some time, they'll have (random numbers) (X-2)% body fat, and (Y+2)% mus

Take a person that with X% body fat and Y% muscles. They start going to the gym; after some time, they'll have (random numbers) (X-2)% body fat, and (Y+2)% muscles. On a scale, their weight will be the same, but in reality, their body composition (and looks) will be considerably different. This is a very important concept. Beside the obvious case where somebody just looks at the scale, and they don't realize their progress, body composition as opposed to weight is extremely important when doing diets, because an improper diet may lead to excessive muscle loss, which is accidentally perceived as a success (on the scale) rather than a failure (because the body composition hasn't changed as intended).