But if they meet in some kind of chamber that's just the right size for their frequency, they can keep on adding together (resonating) to create one big standing wave. When waves meet other waves they add together. And given the right kind of cavity to bounce around in, waves can resonate with themselves too. Water waves, pressure waves (including sound waves), electromagnetic (light) waves - they're all sources of energy that come in a range of frequencies, so they can all cause resonance by pushing other things. In the real world, the regular hits of energy that create resonance generally come from waves. Rhythmic pushing by a responsible adult is one way of adding energy at a particular frequency, but it's not the sort of spontaneous event that really takes off in nature. We naturally push swings at their resonant frequency (say every three seconds), and they store that push energy by oscillating higher and higher. Then you just add energy at that frequency and watch the vibrations build. So to make something resonate, you need to know its resonant frequency (the frequency it naturally vibrates at when you add energy to it, like by pushing, poking or hitting it).
That fundamental law of the playground sums up the basics of resonance - if energy is added to something at its resonant frequency, it can store more and more of that energy by vibrating at that resonant frequency: it's resonating. Kids on swings say 'push me' because they know that with every push they can go a bit higher, as long as the pushes are in time with the swing. (A quick read of Vibrations for beginners should stop those frontal lobes from slumping). It's as simple as pushing a kid on a swing, but to get your head around resonance you've got to read terms like frequency, vibration, oscillation and Hz without your brain clouding over. And everything - everything - in the universe can resonate. Resonance is a way of storing energy by vibrating at a particular frequency. So what is resonance, and why is it so everywhere? Without resonance we wouldn't have music or voices, our whole world would be see-through, and the greenhouse effect would be impossible.
The science version of resonance is behind everything from lasers to MRIs and bridges that vibrate themselves to death*.
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