Theremin is a musical instrument, a product of early 20th ventury Russian research in proximity sensing. It was developed by russian physicist Leon Theremin in 1919 being one of the first electrical instrument. In my opinion it's also one of the coolest musical instrument ever created both due to it's unique sound and the physics behind it.
Theremin produces it's sound using electrical oscillation between two antennas where the antennas and the players body form a capacitor and the sound is controlled by the distance of the players hands from the antennas.
The sound of the theremin is described as wailing and ethereal. Because of this it has been used in many sci-fi and horror movies.
I have always liked the instrument but never seen it played live - until now. Our company gathering secret special program was a theremin player! If that wasn't cool enough her performance was mostly consisting classic movie and videogame soundtracks.
What was even greater surprise than having a theremin performer play at our event, but after she finished she offered everyone cirios a chance to try out the instrument! It is said to be one of the hardest instruments to play and the sound us amateurs could produce was honestly horribly to listen. But I hope my colleagues who had to listen me and all the other curious to play it forgive us for the torture.