An edible substance has negative Calories if the energy used by your body in processing it is greater than the energy derived from the substance itself. It's commonly known -- somewhat incorrectly -- that ice-cold water has negative calories. Celery and other high fiber, non fat foods have them, as do chili peppers by virtue of their spiciness. It's commonly forgotten, however, that stimulants have negative Calories, too.

Stimulants, by their definition, up the body's metabolism. This causes you to burn more Calories during any activity, even sitting still. Most stimulants also cause a cessation of hunger for the duration of, and sometimes for a bit after, their effects, caffeine being a noteable exception. Also, stimulants are not be metabolized the same way as food, and thus have effectively zero Calories. All together, they always give the body negative Calories.

Diet Coke and Diet Dr. Pepper thus are both excellent sources of negative Calories. They make good dietary aids, if you are willing to live with their taste. Ephedrine (Ma Huang, mini-thins, etc.) turns metabolism way up, and suppresses appetite to boot, so it's good too. The various amphetamines not only do as well or better than the others, and they make you feel really good too. MDMA especially can turn off a user's appetite for three or more days -- it was originally developed as a diet aid -- though its metabolism increase isn't as pronounced as some of the others'.

I lost a little over eighty pounds in seven months using the power of negative Calories (and, uh, not eating). They really are a useable side effect of what you eat, and definitely not an imaginary or trivial one.