We've heard an abridged history of the wonderful instrument known as the theremin (and synthesisers in general) and you can even "play" one online using your mouse. But how does the darn thing work? It's simple, but also complicated... but mostly simple.
As SciShow's Hank Green explains in this clip, the theremin operates by using you, the player, as a capacitor:
The theremin doesn't have a whole capacitor in it, instead the theremin has one plate of the capacitor, which is the antenna here. The other plate of the capacitor is my hand. So, the theremin and I, plus the air between us work together to make a full capacitor, which means I control the oscillations of the current.
However, the theremin has an issue -- it can't just pump the current to the speaker, as the frequency is way too high. It uses heterodyning to combine currents and create a new frequency -- one we can actually hear.
Without this, well, the theremin would only have been popular with dogs and last time I checked, the canine film industry isn't very big on science fiction.