Water contains no calories because the human body cannot metabolize it further to get energy out of it. We have no mechanism of converting water to energy.

This would be like putting a hamburger into a car's gas tank. Why doesn't the car run? The hamburger has all sorts of stuff it in. But the car is not designed to use that type of fuel.

a Calorie is a unit of energy needed raise 1 kilogram of water by one degree C. We quantify the amount of energy in molecules by measuring the amount of heat released using a process called combustion. Combustion involves the exothermic reaction of a molecule with oxygen and in the process, the molecule will end up in a higher oxidation state and molecular oxygen will be reduced. If one considers a water molecule, the hydrogen atom is in its most oxidized state, and oxygen is in its most reduced state. It's essentially inert to combustion, but this DOES NOT mean it's energy-less. It just can't be quantified by combustion.

Long story short, water does not have Calories because it can't undergo combustion. However, it is REQUIRED in many metabolic processes and is therefore just as essential and relevant as the combustible fats, carbs, and proteins we take into our bodies.

One last thing: Relying on Calorie content of food is somewhat foolish because this implies that our bodies actually BURN food in order to access the inherent energy.

Oh, and if you're still awake and interested at this point, this website provides a good basic understanding of Reduction/oxidation-http://www.chemguide.co.uk/inorganic/redox/definitions.html#top).

The energy our bodies get from stuff comes from the bonds formed by electrons between atoms in the molecules. Chemistry is all about those bonds, forming them and breaking them. Sometimes breaking a bond liberates net energy, sometimes breaking a bond takes net energy. Since we are not nuclear powered, nearly everything our body does metabolically is related to forming and breaking these bonds. For example converting CarboHydrates (Carbs) to HydroCarbons (Fats) is really just rearranging atomic bonds in molecules.

H2O, water, takes net energy to break the bonds between H and O. So this reaction cannot power the human.

I am confused by this question. The body breaks down carbohydrates, protiens and fats into energy. These are all complex molecules, not single elements. The body does not power its energy on single elements (not even carbon)

I am not sure exactly the cellular reasoning behind this, but I cant imagine biological cells running on hydrogen combustion, or cold fusion or anything (how would you even get energy from a single element? I think you can get energy from a chemical change/reaction but I dont think you can use that on a single element)

Interesting question though. Maybe someone can illuminate the way glycogen, glucose and fatty acids/ketones are used for energy on the cellular level.

Here, I answered a similar question a while ago: http://paleohacks.com/questions/41372/calories-combustion-versus-digestion/41533#41533

tl;dr: basically, water does have energy, everything has energy. What we care about is changes in energy relative to a common "zero". When talking about food energy, we're talking about combustion, and water is one by product of combustion, so water defines the "zero" that everything is relative to.