No batteries, no plugs, no cranking.

They work because while very little power gets to you in a radio wave from your local transmitter, your ear is extremely sensitive to sound, and we can convert the tiny amounts of radio wave power into tiny amounts of sound power, and your ear can hear it just fine.

Crystal radios are very easy to build yourself, from junk you find around the house.

But you can get very fancy, either by improving the circuits to get better volume or more stations, or by building fine cabinetry with brass controls for an old-time look.