Question

Hydrogen is combustible,oxygen helps combustion,but water helps to extinguish fire.

Answer 1

ANSWER--

he characteristics of a molecule is different from the elements it is composed of or other compounds that contains same elements. 
Water contains hydrogen and oxygen in the ratio 2:1 and its properties are different from other molecules that contain hydrogen and oxygen.
Oxygen and hydrogen gas are supporters of combustion but water is not.

Answer 2

since water is made up of two components which are combustible and combustion supporters but when they reacted together to form water the water hasn't any properties of its reactant.

This shows that during chemical reaction New substance with extremely new properties are formed.

Hope u got that