Hearsay. I believed it to be true, but I had never proved it to myself. Just like the sun is 93 million miles away. You knew that, too, did you? Prove it. I can't.
But I was able to prove that I can make a 0.9 volt chemical battery using a copper-plated penny and a zinc-plated galvanized nail stuck in an orange slice. I didn't get the 1.4 volts I expected from the carbon graphite from a carpenter's pencil to that same nail (only 1.08 volts, see below). And the penny to nickel cell didn't work at all in the orange (perhaps I really do need to use vinegar). The rusty iron screw was in the middle as expected. And I found out that a brass screw (made of roughly 65% copper and 35% zinc) acted electrically more like zinc than copper.
|Carbon, copper, nickel, zinc, iron, and orange. DIY battery.|
I think it is important, for everything in life, to always know why you believe what you believe. Don't take someone's word for it. Prove it to yourself.
* - Technically, I created "cells" with metal electrodes and a catalyst. The term battery is more properly two or more cells linked together to give a higher voltage. Your car's 12 volt battery is 6 cells of 2.0 volts in series. Your flashlight "batteries" are 1.5 volt cells.