Eating celery is technically exercise. When you eat celery, you burn more calories than you consume.
A celery stalk contains 6 calories.
Chewing your celery stalk burns: 5 calories/hour
Because of the high insoluble fibre content of celery, you burn even more calories digesting that celery stalk. However, eat just celery and you'll not only be malnourished, but your metabolic rate will slow down to protect your body from your low caloric intake. So, when you inevitably get sick of celery and start eating real food, you'll pack on the pounds faster.