A radio without a battery
When I was about 10 years old, I had a Radio Shack electronics kit. It was a limited model, instead of the solid block with spring connectors, it was a bunch of blocks with spring connectors. It was a bit easier to work with, but like most of those 100-in-1 kits could get complicated. My dad and I build the crystal radio. It used a weird little ear piece; I couldn't understand why at the time. For some reason, I had a long spool of two conductor phone wire. It wasn't the conventional color, I think it was blue and yellow stripe or something similar. We rolled that wire all through the house through the den, kitchen and living room. It was magic to listen through that tiny ear pice and hear a local radio station. How that could work without a battery was so mysterious.