Would you like some fat and protein with that?
When a meal contains fat and protein, the beginning of the small intestine (the duodenum) detects the presence of the fat and protein and sends signals to the stomach to slow it's emptying. Basically, the small intestine says, "hey stomach! Hold on to that food. There is a lot of fat and protein in this meal and they take longer to digestion; I need more time". When the stomach retains its food instead of quickly passing off to the small intestine, we feel fuller longer. Fat and protein are more complicated to digest and absorb then carbohydrates or water-rich foods, so are stomach and intestines are too busy dealing with them than to whine for more food.
Therefore, one of the keys to keeping hunger at bay and to feel more satisfied after a meal (preventing excess snacking): make meals contain some fat and protein.
Note on today's picture: I've being trying to find a way to post the picture above on the blog. I thought today, with my "fries with that" joke was as good a time as ever. Source unknown.