(Updated on 17/9/2013, Tues):
Barton's Pendulums (SPM 2011 P2 AQ4 at pg. 297) : Explanation: "The oscillation of one pendulum - thus, application of external periodic force- causes the other pendulums to oscillate as well due to the transfer of energy through the suspension string. The pendulum having the same natural frequency - i.e. the same pendulum length and pendulum bob mass - as the original oscillating pendulum will oscillate at maximum amplitude due to being driven to oscillate at its natural frequency causing resonance to occur."
For resonance in tuning forks: please see SPM Yr 2012 P1 Q32 @ pg 338.
(Acknowledgment: The above video is publicly viewable at U-tube)