Question

Hydrogen is an explosive gas, and oxygen supports combustion? How is it possible, then, for water, which is composed of hydrogen and oxygen, to put out fires?

Answer 1

its not hydrogen and oxygen that supports combustion. its carbon dioxide and oxygen.

Answer 2

hydrogen burns, burning is combining with oxygen, the result is water. All the chemical potential for the the oxygen and hydrogen have been released in the burning process as energy.
\[2H _{2 (gas)}+O _{2 (gas)}\rightarrow2H _{2}O _{(gas)} + ENERGY _{Bang}\]
to have the arrow go the other way would require the same energy,