There are probably plenty of people who understand this already but based on a few recent posts there are many who don't. If you are watching your weight and doing low-carb it is helpful to know.

Glycogen is energy source stored in muscles. It requires water, which means weight. So if you do a day of very low carb and exercise, or 2-3 days of lower carb, you will deplete glycogen in your muscles. Your weight will go down and *some* of this may be body fat but some or probably most of it is water weight.

So:
If you start a low carb diet and are down 3 pounds first week, then next week is same; you haven't 'stalled', it is probably just glycogen.
If you eat 3000-3500K calories (500-1500 above maintenance but very low carb) and don't gain any weight, it could just be offset due to lower glycogen.
If you have 1 sweet potato or yam (say 150 calories) and all of sudden you gain 1-2 pounds, it isn't because 'carbs are ruining your progress.' It is because the body is using that starchy carbohydrate, in addition to water, to re-fill your glycogen in your muscles. This is a good thing, especially if you have recently exercised enough to break a sweat.

Of course you can also simplify the whole thing and not bother with the scale and instead use other parameters. But for those still watching the #s it is helpful to know what is real weight loss (or gain) and what isn't.