Lettus is such a food, it cost more energy to burn than it gives out.

Celery too, I think. Although that may only be true if your body were to try to digest the cellulose. I'm no biologist though, so don't take my word for it.

Celery definitely has negative calories - ie. you burn more energy chewing it than you take in from the food.

Your question ff is mixed up somewhat. Of course you take in the energy - but you end up burning more than you took in by eating it. So your body is getting the energy it needs temporarily, but needs more to sustain your chewing.

So in answer to your question, if you are eating nothing apart from negative calories, your body uses its calorie store, ie. your fat, hence why its good to eat celery when you are on a diet.