Food Cravings?!

Okay, so I'm going off of something that I experience like every day. I know we will probably get to this part of the book eventually but I figured I'd just do this anyways. So every day a person needs to eat food in order to survive. Junk food is naturally the humans' favorite type of food to pig out on. What I want to know is, why is it that when we are done eating and aren't hungry any more, do we still want to eat food. It's like I know I'm not hungry because I just ate but then you think of something like a fruit roll up and you're like that sounds so good right now. So why does our brain tell us that we need to eat it even though we know we don't. It's like we are addicted to food and just can never stop eating it. I'm sure that we learned about the part of the brain which causes this but i can't remember at the moment. Then there's the thing with women and loving to eat chocolate, but i guess that can be for another time.