"See the Martian movie - is he right? Take oxygen, add hydrogen, burn?"

As kids--13 years old or so--we used to take dry cells and separate hydrogen from oxygen by putting the ends of wires from the positive and negative ends into water. (I don't recall from which the O or H came off as gases.) We collected the gases in overturned gallon jars. While that was going on we took the screw tops of the jars and put a spark plug in so that later, when we figured we had enough of the gases collected in the separate jars we screwed the tops back on and made them as air-tight as possible. We righted the jars and put them far away from us, probably the only smart thing we did. When we hooked up the wires again to a dry cell so we could spark the plug, one of the jars would explode. Possibly both jars would but I don't recall. I expect Steve would know. However, as Steve said earlier, getting the gases back to gether again requires much more skill(?) than we had as young teenagers, and I'm not sure we would have been all that interested because we just wanted to make things go boom. As noted earlier, it requires energy to separate the gases that form H2O. I expect it would require energy to put them back together again. We went through dry cells like crazy. We only did it twice because we couldn't afford to keep buying batteries. It was likely me who suggested we use 110-volt house current to do it but that blew fuses and I do remember getting what-for because of it. My mother wasn't happy tripping over things in the dark and she let the three of us (two friends and me) know about it in no uncertain terms. Thus ended our promising careers in the explosives/demolition industry.