What body weight is actually made of
Total body weight is the sum of several compartments. Fat mass includes essential fat needed for normal function plus storage fat. Skeletal muscle is the contractile tissue that moves the body. Organs and bone make up a substantial fixed share, and body water, both inside and outside cells, is often the single largest component by mass.
Because these compartments differ in density and function, weight alone is a blunt summary. A simple model splits the body into fat mass and fat-free mass, while finer models separate muscle, bone, and water.
Why same weight can look different
Muscle is denser than fat, so it occupies less volume per kilogram. Two people at identical body weight and height can therefore look very different depending on how much of that weight is muscle versus fat. This is also why the scale alone can be misleading during training: muscle gain and fat loss can offset each other, holding weight steady while shape changes.
Water shifts add further day-to-day noise to the scale. This is general educational information, not medical advice.