What actually is a calorie?

So many people try and avoid eating ‘calories’ but what actually is a calorie and are all calories the same………a calorie is simply a unit of measurement for energy. So in theory 10 calories from candy has the same amount of energy as 10 calories form celery however we do now know that our body uses these calories in very different ways (which we didn’t know when calories were ‘invented’). Which means maybe we will find a new way to measure how much energy we eat in the future……..