Glycogen is another aspect here. We store a sugar called glycogen in our liver and muscles for sudden work demands. Each gram of glycogen binds to several grams of water. When someone goes on an extreme low carb diet they use up their muscle glycogen and release all the bound water but do not eat a surplus of calories or carbs to replenish it. Sometimes people on a strict diet find that they gain 10 lbs after a single large meal. They didn't gain 10 lbs of fat which would require a calorie surplus of 35,000 kcal but rather they replenished muscle glycogen and absorbed water associated with it. This kind of water weight does not come back when you drink water; it comes back when you eat extra calories and carbs.
Exactly what rotten sheep said^. I mean when you sweat, and get dehydrated in general, your body loses all of the water. But as soon as you rehydrated all the water weight will come back. "normal" weight loss involves losing actual fat, and muscle in some cases. These changes aren't necessarily permanent but they are longer lastings than losing water weight.
Losing "water weight" is dehydrating. And water weight will come back as soon as you drink sufficiently. There is simply less liquid stored inside you body.
Losing "normal" weight is more difficult. It's actually losing fat. It lasts longer.
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