It's true that a few foods, such as grapefruit and celery, contain fewer calories than it takes to digest them. So, when you eat these foods, you actually burn more calories than you take in. Thus, the term "negative-calorie foods."
At one time or another, we've all been promised this: "If the amount of calories we eat is less than the amount of calories our bodies use, we will lose weight." I'm sorry to say that this promise is false.