As if we needed another reason not to like celery, or to stand firm in our knowledge that it is a vegetable concocted in the lower pits of the Inferno, it turns out that celery has negative calories. That's right. It costs more energy to digest it, than you receive. It's not even real food! In my book, the only proper place for a stalk of celery, is lying on the bar next to my bloody mary.

I thought this was an urban legend, but Snopes confirms it.