All food has calories -- they can't help doing so. But, some foods take more calories to actually eat than they provide your body be having eaten them. So, the term "negative" comes from the overall net result. Chomp for 20 minutes eating a food containing 45 calories and you'll probably burn 50 calories in the process.

There have been books and countless studies done on "negative calorie" eating, and there are some people I know personally who base a good portion of their diets on eating foods that are tasty, but that require more effort to eat than the caloric count being provided. Hence, weight loss in an easy-to-swallow package.

As one would expect, most "negative calorie" foods fall in the vegetable and fruit ranges -- so no pan-fried bacon or steaks here.