[MarianaM (OP)]

Harriet asks...

If hydrogen and oxygen are both flammable, why doesn't water burn?

Can you help?

[Hayseed]

Water is the ash of burning hydrogen and oxygen.

[Bored chemist]

Oxygen is not (normally) flammable.

[Janus]

Just to add to what has already been said.
If you burn hydrogen in an oxygen rich environment, you get water and a lot of energy released,  The hydrogen and oxygen have combined to form a molecule.    The only way to re-burn it is to separate the hydrogen and oxygen into separate gasses again, but you can only do that by putting back the energy you got out when you burned them together.
Since you can never make 100% efficient use of the energy you got out (you'll always end up wasting some of it) and there will always be some waste when trying to separate them again,  repeating the process over and over again ends up in a net loss of energy.

Water doesn't burn for the same reason that salt (Sodium chloride) doesn't kill us when we ingest even though its elemental components are a metal(Sodium) that is so reactive that, in its pure form, it would cause severe burns just from contact with your mouth, and a poisonous gas(Chlorine). 
But together, they make up table salt, which we put on and in our food. 
While alone, they are very reactive, together, they cancel each other out (essentially they cling to each other so tightly that they aren't free to react with anything else.)

[JazzHandsMafia]

'Burning' is actually a redox process.
The hydrogen and oxygen in water are fairly stable redox wise, but only in comparison to other natural compounds.
Thanks to this, the oceans aren't literally on fire.
However, strong oxidizers do, in effect, 'burn' water.
A dramatic example is chlorine trifluoride: ClF₃, CAS 7790-91-2.
Upon contact with water (or almost anything else), violent reaction occurs.

TL;DR:
Anything can burn with the right incentive.

Note: I wanted to embed a good youtube vid showcasing the stuff, but can't link currently. Look up Chlorine Trifluoride - it's fun!