We have all been there. It's 3 o'clock and you've been hard at work. As you sit at your desk, a strong desire for chocolate overcomes you. You try to busy yourself to make it go away. But it doesn't. In fact, the more you try to NOT think about a square of deep, dark chocolate melting in your mouth, the more you want it. You can even smell it!

Here is another scenario. Perhaps you are not feeling well. You’re coming down with a cold and feel rundown. The only thing you want to eat is a big, bowl of chicken soup, like your mom used to make when you were sick as a child.

A food craving is a strong desire for a specific type of food. And they are normal. Most people have them even though what we crave might be different. One person might crave sweet chocolate, while another might crave salty potato chips.

Sometimes the foods we crave are not super healthy ones but rather fatty or sugary foods. Or sometimes we crave foods from our childhoods, like American meatloaf, Polish pirogues or Vietnamese pho. Other times the cravings may be for something healthy but very specific, such as Japanese sushi or Korean kimchi.

But where do food cravings come from? And how are they different from hunger?

Scientists at the website How Stuff Works compare hunger and cravings this way. Hunger is a fairly simple connection between the stomach and the brain. They even call it simply "stomach hunger."

The website explains that when our stomachs burn up all of the food we have eaten, a hormone sends a message to one part of the brain, the hypothalamus, for more food.

The hypothalamus regulates our most basic body functions such as thirst, hunger and sleep. The brain then releases a chemical to start the appetite. And you eat.

A craving is more complicated. It involves several areas of the brain. These areas make up the reward center of the brain. A craving can also be tied to our mental state and memory. So, some scientists call food cravings "mind hunger."

Scientists add that while hunger is a function of survival, cravings are not. People often crave foods that are high in fat and sugar and not foods that can keep us alive.

Foods that are high in fat or high in sugar release chemicals in the brain. These chemicals give us feelings of pleasure and even mild and temporary euphoria -- much like a drug.

Back in 2004, researchers at the Monell Chemical Senses Center and the University of Pennsylvania School of Medicine studied food craving and the brain.

They used functional magnetic resonance imaging or, fMRI, to show that food cravings activate brain areas related to emotion, memory and reward. These are the same areas of the brain activated during drug-craving studies.

The lead author of the study was Marcia Levin Pelchat, a sensory psychologist. She says that the finding of the study is "consistent with the idea that cravings of all kinds, whether for food, drugs, or designer shoes, have common mechanisms."

They work the same way in the brain. Food cravings, drug addiction, and addictive behaviors such as gambling and over-shopping follow similar neural pathways.

Studies show that our mental state affects our food cravings but not really our hunger levels.

Also, our food memories affect what we crave and when. For example, if a child is given sweets when he or she feels sad or upset, that may lead to food cravings for sweets later in life. The reward system in the brain may lead us to seek out familiar pleasures.

In a 2007 study, researchers at Cambridge University found that “dieting or restricted eating generally increase the likelihood of food craving.”

So, the more you deny yourself a food that you want, the more you may crave it. However, fasting is a bit different. They found that eating no food at all for a short period of time, lessened food cravings.

So, the next time you crave food from your childhood or have a hankering for something very specific, know that your brain may be more to blame than your stomach.