For the same reason that Sodium and Chlorine, both deadly poisons to humans, can combine to create NaCl -- table salt. Different combinations of elements (and in differing proportions) create different compounds with different properties.

1) "Water (H2O, HOH) is the most abundant molecule on Earth's surface, composing of about 70% of the Earth's surface as liquid and solid state in addition to being found in the atmosphere as a vapor." Source and further information: http://en.wikipedia.org/wiki/Water_(molecule) 2) "Highly-concentrated sources of oxygen promote rapid combustion. Fire and explosion hazards exist when concentrated oxidants and fuels are brought into close proximity; however, an ignition event, such as heat or a spark, is needed to trigger combustion. Oxygen itself is not the fuel, but the oxidant." Source and further information: http://en.wikipedia.org/wiki/Oxygen Hydrogen, on the other hand, is the fuel, when water appears by burning hydrogen with oxygen. So water cannot burn, because water is already "burned hydrogen". Further information: http://www.answerbag.com/q_view/45821

Not only that, add another Oxygen atom and you've got Hydrogen Peroxide (H2O2), witch in it's pure state will burst into flame upon contact with organic matter. Everything in the universe is comprised of some combination of elements, and there's only about 120 of them yo...

The process of burning takes certain compounds and combines them in ways that cause them to release energy. Thus, the products of burning (the "ash") are in a lower energy state than the compounds that were before they were burned. This is why they burn, to get to the lower energy state. So, the products of burning aren't usually flammable themselves because they have already given up what energy they can.

Actually it CAN IGNITE if the fire you feed it to is hot enough to decompose it, OR if the fire is caused by a material which itself reacts with water. Two cases which I have experience with. 1. I once had a problem with a coal burning fireplace . The soot, essentially unburned carbon, which deposits in the chimney will eventually ignite if not swept out regularly by a chimney sweep (see Mary Poppins). My chimney in a rented house did catch fire for that reason. I decided to put the fire in the fireplace out with water. The heat of the coal fire boiled the water and steam went up the chimney. The steam (water vapour) encountered the fiercely burning carbon up the chimney and decomposed to free hydrogen which promptly exploded. In the old days before natural gas, this was in fact one way of making fuel gas but under somewhat different and safer conditions. Today it is known as producer gas and contains both hydrogen and carbon monoxide, both of which burn. 2. I had an aluminum can filled with potassium metal stored in paraffin oil in my shed for many years. I forgot about it. Gradually the can corroded and the oil leaked out. The bare potassium metal reacted with the air and burst into flame and set my wooden shed alight. The local fire brigade sprayed water on the shed to put the fire out. When it hit the burning potassium it reacted to give hydrogen which promptly exploded. That is why Firemen MUST ALWAYS be aware what kind of materials are in a fire because water can end up feeding the fire rather than putting it out. The only reasons water puts out a fire is (a) it cools the fuel below its ignition point, and (b) by forming a layer of steam if chokes off the air supply. Neither of these work if the water itself can be attacked by the components or heat of the fire.

Chemically combining something with oxygen is often called "burning", or "setting fire to it", or "pyromania". Things that are completely burned do not burn any more. Water is completely burnt hydrogen. You can see in the formula that the hydrogen has combined so well with the oxygen that they formed one formula H2O. See also http://www.answerbag.com/a_view/99896