Normally, a trackball floats around on a set of wheels, which detect its movement and translate that into pointer-control on the screen.
Jacek Fedoryński created a trackball that has no external sensors at all. Instead, the magic happens inside the trackball itself — it's got tilt sensors and a gyroscope feeding live data to a microprocessor that calculates the ball's orientation. The result is a vaguely magic-looking object: He just plops the ball into a plastic stand, yet it moves the mouse.
As he admits, it's not super usable because his 3D printed ball isn't quite smooth enough to roll fluidly — but it's a cool demo nonetheless.