Resonance is easy to detect and explain. Resonance occurs when an object vibrates with its natural frequency due to input of energy.

Clear demonstrations are possible. Take two identical tuning forks and strike one with the other close by. On a count of three, hold the prongs of the tuning fork which was struck. What do you feel? What do you hear? How could you make this into a whole class puzzle activity? Observers or participants are certainly intrigued by this activity.

The two tuning forks must be of identical frequency for this to work and ideally should be mounted on wooden sounding boxes.

An analagous phenomenon is Bartons pendulums in which pendulums of identical frequency connected through a flexible string exchange energy while others cannot move effectively due to mismatched frequencies: like pushing a swing with the wrong tempo, some pushes slow down the motion and others miss the swing altogether.

Only those of identical frequency can exchange energy.

This can be set up by tying a string between two stands. Tie four or five lengths of string to the string so that they hang down. Tie weights onto each one and ensure that two only are the same length. Set one of the two identical pendula moving and watch what happens.

The simulation below allows the input of energy and shows how this can give rise to resonance but only for the correct frequency pendulum.