I used to teach a college electronics course. One of the labs was a crystal radio. On the side, I had an old Philco car radio where you could still SEE the damn germanium diode demodulator. (You could also see the FM stuff, but that was over my head!
). When exploring the car radio, I had to first show them how to tune the radio stations, and then show them what you were actually doing -- you were changing the preset values of a variable capacitor.
I also had an old TV tuner. Remember tuning stations on those? In that case, what you were actually doing was moving iron cores in and out of an inductance coil as you turned the tuning knob.