This is definitely the most citrusy survival skill you'll ever learn. It's the perfect technique for anyone who happens to get lost in the woods with a lemon, copper clips, zinc nails, wire, steel wool, and dry tinder. You can use chemistry to make a lemon generate five volts of electricity, and use that electricity to set the steel wool on fire.
The first step is to warm the lemon up, rubbing it with your hands. Once you've got it nice and warm, stick copper clips in a line down its side, spaced about a centimeter apart. Then, a few centimeters across from each copper clip, stick in your zinc nails in a line.
Now that your lemon looks a little bit like a porcupine, start with the wire. Connect the first copper clip to the second zinc nail, connect the second copper clip to the third zinc nail, and so on, leaving the last copper clip and the first zinc nail free. Connect each of these to a longer piece of wire. Once you've done this, you've got yourself an electrically charged lemon. The reason this works is because the lemon juice serves as an electrolyte, which carries a small electric charge.
Now all you've got to do is give it something to light. Throw some dry tinder on the steel wool—the guy in the video uses bits of toilet paper, but twigs and leaves should work just fine. Touch the ends of the loose wire to the steel wool, creating a short circuit that will rapidly heat the wool. Once your tinder lights, you've got yourself a lemon fire.