The scale is the dreaded measuring device most commonly used to check changing body composition; that is, are you getting fatter? It's not a good measure, though, since it doesn't really tell us anything about body composition--the ratios of fat, muscle, bone, and water that make up total body weight. Water can be particularly confounding since one cup weighs about half a pound, so drinking a beverage or using the bathroom can immediately change results in a measurable way. What we're really interested in is body fat percentage--that's the most useful measure of body "comp."
If the scale isn't up to snuff, what is?