HHO is the product of water electrolysis- separating the hydrogen and oxygen in water. Several years ago, I was laying in bed, unable to sleep, and I thought about a hydrogen-powered, internal combustion engine that used water. By separating the hydrogen and oxygen in water, you have very flammable gasses. When reacted, it goes back into plain ol' water, as evidenced in the emissions of hydrogen-powered vehicles.
Someone has kind of got the idea here:
HHO car adaptation