To make water, you need a lot of hydrogen. There's very little of that in the atmosphere. The only viable source of it is...water. We can make hydrogen from water by splitting the water molecule. Unfortunately, this takes a lot of energy. Though it's doable, a really large-scale economical method has yet to be devised. I think that one day we'll be able to use solar energy to power water-splitting. We can split water on a very small scale in the lab, and your grandson may well have seen it. We called the piece of kit Hoffmann's Voltameter when we were at school. You fill it with acidified water and apply a current to the platinum anode and cathode. The water is split and you get oxygen at one electrode and hydrogen at the other. There is twice as much hydrogen, nicely demonstrating H2O. The best way to get pure water is to purify water from reservoirs, rivers or the ocean. Getting pure water from sea water requires a lot of energy, and only a few countries do it, mostly those with very dry climates. If we could get hydrogen cheaply it would solve our energy problems and probably stop global warming, because when you burn it it releases a lot of energy, and all you get as waste product is water. It's a bit hard to handle, though, as it is very explosive in the presence of oxygen, but, one day, I think it will be a major energy source.