A number of Arduino fans are currently experimenting with an interesting quirk of the platform: the ability to detect motion with nothing more than a twisted length of wire inserted into an analogue pin and a ground pin.
Motion detection is handy in a variety of projects, and there are a number of ways to do it: you can use passive infra-red sensors, webcams linked to computer vision tools, or even affordable thermal sensors. Few, however, are as cheap as a length of wire - but this particular phenomenon, brought to our attention by Hackaday, is notable for more than simply being affordable: nobody seems able to quite agree on how it works.
Writing up experimentation on the subject by Connor Nishijima, and linking it to earlier work on the same subject, Hackaday has been unable to decide on quite how the system works. Suggestions in the comments have ranged from the human body acting as a non-contact capacitor to the wire being affected by static charge, with others pointing to the wire acting as an antenna in much the same way as a Theremin.
Few, however, have ventured an explanation for Connor's other observation: of all the microcontroller boards he has tried, the trick only appears to work reliably on the Arduino.
I'm coining this phenomenon as "Capacitive Turbulence", which sounds fancy enough to me. To my best guess, this is possible via changes in capacitive coupling between myself and the ADC of the Arduino Uno. Logging these changes over time results in a graph that directly pertains to the nearby motion! Again, this only seems to detect living things, and possibly strong EMF disturbances like lightning or AC mains.