What happens in your body when you eat fat or carbs? Here are more awesome and simple illustrations from Dr. Ted Naiman, an "über-geeky mechanical hydraulic model of metabolism".
Above you can see what happens when you eat a low-carb, high-fat diet. Your body gets its energy from the vast fat stores (in blue, to the right).
But what happens when you add carbs?
Some carbs
A standard American high-carb diet
In this third image the body mainly runs on carbs, and the fat stores start filling up with... fat. If you make sure to eat regularly, never fasting, you're pretty much guaranteed to end up with quite large fat stores eventually.
As you might already have guessed - a great way to avoid the dreaded case #3 is to eat a real food, LCHF diet.