I just love this one, it’s not what I was expecting (I was thinking they were going to use the potato itself for fuel since it’s very starchy), and I’m really

impressed that they managed to get enough juice out of a potato-based battery to start a fire with, very nifty. What’s happening here is that the fluoride in the toothpaste and salt are forming an electrolyte which is reacting to the copper and zinc (one wire is copper, the other is zinc-coated: this is key, make sure you get this right) such that each wire is

an anode and cathode, waiting 5 minutes allows the chemicals to react and build up an electric charge so the potato thing is now essentially acting as a battery, and then the heat produced by shorting the two wires together is igniting the cotton. You can get a similar reaction with a 9-volt battery and some steel wool (video of that is below, very cool and it takes far less time and prep than the potato thing, so that one is for the lazy people and this first one is for the DIY nuts), here’s the first video on how to make fire with a potato:

Ok, here’s the video on how to do the 9-volt battery and steel wool thing–personally I think this would be a great little kit to

carry around as a backup fire-starter if you were camping or out and about in the woods, you could very easily just stick the 9-volt and the steel wool in some watertight bags (separate bags, obviously, because you don’t want the leads on the battery contacting the steel wool while it’s stored). Here’s the video:

Oh, by the way, try touching a 9-volt battery to your tongue–it’s not enough to hurt you or anything, but you will feel a bit of a tingle due to the conductive nature of your saliva

Further Reading and Additional Resources

Excellent scientific explanation for why the 9-volt steel wool thing works at MadSci.org: How come when you rub steel wool to 9 volt battery it starts on fire?