Using Water for Fuel?!

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or "Brown's Gas".... I've noticed this "Alternative fuel" process popping up on the interweb...I see various people will sell an ebook listing components you'll need to build the gadget for around $100 (not including the parts!) and others will sell you a complete unit for something like $750... From what I read,you are building or buying basically a high school chemistry experiment that you're going to bolt onto your engine...tapping electricity from your car battery to electrolyse water into hydrogen and oxygen which is then fed into your engine's air intake.. They claim impressive figures for what it can do for your car's economy,either petrol or diesel.. Has anyone You know built or bought one of these things,and most importantly,does it actually WORK?! (Also,please correct me if I have my facts wrong!)