In the presence of an electromagnetic wave, a resonant circuit converts between magnetic and potential energy in a pendulum-like motion. If the wave happens to oscillate at the natural frequency of vibration of the circuit, it starts a vicious cycle of exponential increase. Such vicious cycles are present in the shattering of a glass by an opera singer or the collapsing of a bridge by a particular earthquake vibration. Sonic, seismic and electromagnetic waves whose frequency is in syntony with the receiving body put escalating forces in motion. Such is the magic behind a tuning dial: a tiny body, adjusting its resonant frequency to one particular radio wave to vibrate in tune, allowing it to escalate beyond all other frequencies.