Here, 3 sets of pendulums of different lengths are suspended freely from the hinges. Choose the longest pendulum and oscillate it slowly. For the next trial, choose a pendulum of different length and oscillate it.

What did you observe? 
When you oscillate the longest pendulum, another pendulum of same length starts to oscillate and attains the same frequency.

The science behind it :
Resonance causes the oscillation of the pendulum having same length.