Scientists are starting to unravel the mystery of moreishness - why some snacks seem impossible to eat in small amounts.
It is the phenomenon that explains why it can be so difficult to dip into a crisp packet without polishing off all the contents.
Likewise, a single bite of chocolate may prove waist-expandingly fatal. Some might call it greed, but another name for such behaviour is "hedonic hyperphagia".
"That's the scientific term for 'eating to excess for pleasure rather than hunger'," said expert Tobias Hoch, who presented findings from a study of hungry rats to the annual meeting of the American Chemical Society in New Orleans.
"It's recreational over-eating that may occur in almost everyone at some time in life. And the chronic form is a key factor in the epidemic of overweight and obesity that here in the United States threatens health problems for two out of every three people."
Dr Hoch and his team from the University of Erlangen-Nuremberg in Germany scanned the brains of rats as they ate crisps - called "potato chips" in the US - a powdery mixture of fat and carbohydrates, or ordinary chow pellets.
The rats were far more keen on the crisps, despite the fat and carbs mixture containing the same number of calories. Standard pellets were the least popular food.
"The effect of potato chips on brain activity, as well as feeding behaviour, can only partially be explained by its fat and carbohydrate content," said Dr Hoch. "There must be something else in the chips that make them so desirable."
High levels of fat and carbohydrate had been thought to send pleasing messages to the brain, leading people to gorge on calorie-packed snacks. The magnetic resonance imaging scans showed that reward and addiction centres in the rats' brains were most active when they ate crisps.
Food intake, sleep, activity and motion areas were also stimulated differently by crisps compared with other food. Pinpointing the molecular triggers in snacks and sweets that stimulate the brain's reward centres could lead to the development of new drugs or food additives that combat over-eating.