Time of Day
Through routine, we condition our bodies to expect breakfast, lunch and dinner at the same time each day, says Randy Seeley, a professor of psychiatry at the University of Cincinnati." Part of the reason you're hungry at noon is because that's the time you've eaten for the last 100 days."

Sight
Research using MRIs shows that brain patterns of people viewing photos of foods they like and foods they don't like are "very different," Seeley says. "The body anticipates when food is about to enter the system." And that's why your mouth starts watering when you see Mom's pie.

Variety
Even after eating a large meal, we often "make room" for dessert, because a desire for sweets hasn't been satisfied. Ann Gaba, a registered dietitian at New York Presbyterian Hospital, says that sometimes all it takes is a bit of fruit in a salad during a meal to curb a sugar craving.

Smell
Scent is one of the key ways we cue our bodies that food is near. Once the trigger goes off, it can induce the insulin secretion that makes us think we're hungry. Says Sharron Dalton, a nutrition professor at New York University: "Smell and sight alone activate the appetite cascade.".

Alcohol
Drinking has not been scientifically proved to stimulate appetite, but too much beer, wine or liquor can impair judgment, causing us to eat more. "Most people who are on a diet will say it's a lot harder to push themselves away from the table if they've been drinking," observes Seeley.

Temperature
The colder the temperature, the more people tend to eat, which is why restaurants often keep thermostats low. "Your metabolism drops when it's time to eat, and eating warms you up," says David Ludwig, professor of pediatrics at Harvard. "Heat is a satiety signal."

Refined Carbs
After a meal heavy in refined carbohydrates, like white pasta, the body may crave food again within only a few hours. These foods cause blood sugar to drop, and "when our blood sugar is crashing, we're going to be a lot more interested in food in general," Ludwig says.

source: http://www.time.com/time/photogallery/0,29307,1626481_1373603_last,00.html