Water, like all liquids, is a fairly dense substance. It can't be compressed down like fluffy bread or other starches—which can pack into the stomach in large quantities—so it doesn't take a ton of liquid to trigger those stretch receptors. Of course, it won't make you feel full in the long term. Water doesn't contain nutrients and it doesn't need to be broken down, so none of those digestive steps that prompt feelings of satiety happen. The full feeling you get from water will only last a short time, but if you drink a few glasses about 10 minutes before the meal, you'll go into it less hungry and will probably end up eating less. By the time your stomach has figured out that you were tricking it with plain ol' H-2-O, it will already be contending with your turkey and stuffing, so you really will feel satiated.