So you've got a car powered by hydrogen. Totally environment friendly - so I've read - water comes out the other end of the process. (Not seen one for real, just read about them!)
Now water is hydrogen plus oxygen, as you know, so some'ow (don't understand this bit!) the hydrogen you've 'used' is still there in the water. Wouldn't it be possible to separate the two back out again so they're both gases and re-burn the separated-out hydrogen in the separated-out oxygen? And if you get MORE water coming out the other end of the process, couldn't you just set up a loop so the new water would go back to the separator, separated back out and re-reburned and so on? Surely you'd end up with an infinite power-loop, constantly reusing the same hydrogen?
Now there's GOTTA be a problem with the above, or someone would've thought of it by now! But if by burning hydrogen in the first place you're getting water coming out the other end, I'm not seeing what the problem IS! (And if you've got a separator, the above aside, couldn't you just power everything with purified water?)
What am I missing here? (Apart from a brain!)