I really loved those grade school experiments like making crystal radios and making your own electric motor. This instructable is along the same lines:
I guess you’re making a dry cell (zinc-carbon) battery, although I wonder if it would be even more effective if you could suspend the electrolyte (the vinegar) in a gel. The cardboard in this case sort of acts like a gel, but if you could make an acidic gel, that coat the interior of the cardboard lattice and wouldn’t dry out, although the example shown here seems to be working for days.
And it, of course, reminds me of McGyver’s potato-powered clock.