PHYSICS
'(<a href="http://www.j-archive.com/media/2009-03-05_DJ_24.wmv">Sarah of the Clue Crew demonstrates pedulums in action.</a>) Two of the three pendulums have the same resonant frequency, in this case, the same length; when one swings, the matching one <br />will begin to swing, too,  which is known  as this type of vibration'
sympathetic vibration