sympathetic resonance

A tuning fork doesn&rsquo;t teach the piano to sing. It reminds the piano what key it&rsquo;s already in. Strike a tuning fork and hold it near an open piano. Don&rsquo;t touch the strings. The fork vibrates at 440 Hz — concert A — and if the piano is in tune, the A strings start vibrating on their own. No contact. No mechanism beyond air moving at the right frequency. The fork doesn&rsquo;t make…