Celery is known for being extremely low in calories so much so that you burn the calories off just by eating it. The energy required to chew, swallow, digest and eliminate celery, called the thermic effect, is about 15 percent of its caloric value. So, if you eat 2 stalks that contain a total of 12 calories, you would burn close to two of those 12 calories processing the celery.
http://www.livestrong.com/article/299384-how-many-calories-does-eating-a-celery-stick-burn/
