The effect of calorie gain or loss on weight

We hear about calories all the time.How many calories are in this cookie?How many are burned by 100 jumping jacks,or long distance running,or fidgeting?But what is a calorie, really,and how many of them do we actually need?Calories are a way of keeping track of the body's energy budget.A healthy balance occurs when we put in about as much energy as we lose.If we consistently put more energy into our bodies than we burn,the excess will gradually be stored as fat in our cells,and we'll gain weight.