File this one to the interesting but not super useful category (unless you carry a lot of copper tacks, nails, wire and lemons in the field). However, it’s an interesting demonstration of ingenuity, using the acidity of a lemon and common items to create an electrical current.
For a more practical use of this technique, you just need a battery, wire, and steel wool to set tinder ablaze.
Editor’s Note: Since publication, it’s come to light that the “battery” as designed in this video will not produce enough electricity to start fire. However, similar techniques will create significant electrical current, such as this one by Crazy Russian Hacker.