Happy Parent, Happy Child

Two pianos were sitting next to each other in a room.  A person sitting at one piano pressed one key,
causing a tone to fill the air.  
Instantly, the string responsible for creating the same tone on the
OTHER piano began to vibrate.  This isn’t
the start of a joke, but an experiment that has been tested by physics
experts.  The tone created by the first
piano is a wave of vibrations that are