There's a reason why it's called a Happy Meal − junk food is extremely pleasurable. And that's not an accident. Junk food has been carefully engineered by food scientists and snack manufacturers to hit the "sweet spot" that keeps us wanting more, and more, and (gobble, gobble) more.
It starts in the mouth. One of the distinguishing characteristics of all great junk food is mouthfeel. Cheetos are famous − or infamous, depending who you ask −for delivering explosive cheesy flavor without the heavy, greasy mouthfeel of actual cheese. Instead of tasting like the salty fat bombs they are, Cheetos literally melt in your mouth.
One food scientist calls this vanishing caloric density [source: Moss]. Your brain relies partially on mouthfeel to determine how many calories are in a particular food. By melting into nothingness, Cheetos trick you into eating the whole bag.
Fast food cashes in on the same mouthfeel science. A meal of a hamburger and fries is undeniably fatty, and your mouth knows it. But food scientists have discovered that the potentially unpleasant mouthfeel of greasy fries is perfectly balanced out by the astringent quality of carbonated beverages [source: Cuda Kroen]. And bonus points for balancing salty with sweet!
The food industry spends tens of millions of dollars a year researching and developing the ideal combinations of mouthfeel and crunch that keep us coming back for more. Apparently there's a $40,000 artificial chewing device that calculates the exact amount of pressure required to devour a chip (the perfect crunch: 4 pounds per square inch) [source: Moss].
Junk food science also owes a lot of credit to evolutionary biology. The brains of modern humans evolved in a place and time when daily life was consumed by the search for more calories. As omnivores, we can physically digest a wide variety of foods, but not all foods are calorically equal. Meat, for example, delivers many more calories per bite than leaves.
To feed ourselves most efficiently, the brain evolved ways to quickly identify the most calorie-dense foods. And it still does, even though food scarcity is no longer an issue. When hunger strikes and we stand in the kitchen choosing between an apple and a bag of potato chips, a region of the brain called the ventromedial prefrontal cortex screams, "Chips, you fool! We could die tomorrow!" [source: Tedesco].