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Image Credit: thegoodsurvivalist.com
With something as simple as an old ice cube tray some soil a few bolts and copper wire you can get about 5 volts of electricity. This is enough to power a few LED lights, calculators and if you are really tech minded even a mobile or USB device.
Imagine if you scaled this up to say a few hundred of these? The power could be usable and if captured could be stored into batteries. This method is just perfect for experimenting with kids and if we are honest and just sit down and think about things long enough a real viable solution for off-grid power.