When you fill your car up with petrol, you are filling it up with energy that you can use to go places.  Similarly, the food we eat provides us with energy that our bodies can use.  However, to think that the kilojoules or the calories in the food that we eat becomes usable straight away is oversimplifying matters.  Just having the raw fuel - even if it is of the best quality - doesn't necessarly mean that you can extract and use its energy, as Tat Loo writes about here .