So my grandson, who is 11, asked the other day: If we're in such dire need of water, why don't we make it ourselves? After all, it's a simple molecule - some oxygen, some hydrogen, mix `em together, and bingo.

I did not have an answer. I think I remember that those elements are quite combustible, but my recollection of high school chemistry is too vague to tell him anything useful. Who can help?