In a room with two similarly tuned pianos, if you strike a key on one, the

other piano will faintly create the same sound without being touched.  The

vibration of the string that was struck will cause its counterpart in the other

piano to begin to vibrate.  Something else in the room will begin to move

without you touching it—your inner ear.

      Your ear has a similar string-like structure that also begins to vibrate. 

The sound that is created in your ear travels to your brain, which gives you

your hearing and a memory of what you have heard.