You can make a fun spike mic to listen to people in another room with old parts.
Take an old phono cartridge and carefully attach a metal nail to the tiny stylus end (where the diamond is).
Then plug it into an old phono amp as usual, and listen with headphones.
If you drill a hole on your side of the wall, big enough so the nail doesn't touch the edges, but lightly presses against the wall of the next room, you might be able to hear what's going on.
(Don't use this illegally!)