Crystal radios are simple radio receivers which require no batteries or other source of electricity.
A crystal radio can be made with a few hand made parts, like an antenna wire, tuning coil of copper wire, crystal detector and earphones. Because crystal radios are passive radio receivers, they are technically distinct in many respects from ordinary radios containing active powered amplifiers. This is because they must receive and preserve as much electrical power as possible from the antenna and convert it to sound power whereas ordinary radios amplify the weak electrical energy "signal" from the radio wave.
Crystal radios can be designed to receive almost any radio frequency since there is no fundamental limit on the frequencies they will receive. The most common crystal radios are designed for the AM Broadcast Band and the 49-meter international short wave band, partly because the radio waves are stronger in those bands. Although crystal radios are designed to detect AM, they also frequently detect FM fairly well which is in the 100 MHz range.