Steven Witherly, author of "Why Humans Like Junk Food," "ticked off a dozen attributes of the Cheetos that make the brain say more. But the one he focused on most was the puff’s uncanny ability to melt in the mouth. 'It’s called vanishing caloric density... If something melts down quickly, your brain thinks that there’s no calories in it . . . you can just keep eating it forever.'"
From a very long NYT Magazine article titled "The Extraordinary Science of Addictive Junk Food."