Watching this cube slowly turn itself inside out as its internal gears mesh together is a mesmerising and puzzling experience. It looks like it should never work, but it does.
I have a degree in engineering. Hell, I have a PhD in engineering. I (kind of) know how it works. But it still looks like magic to me.
It's of little practical use, sure -- but it does illustrate how powerful 3D printing, the process used to create this thing of wonder, can be. The beauty of 3D printing is the way it can turn a theoretical, abstract idea into something tangible without the need for craft skills. In fact, in that vein, over at Thingiverse there's a bit of a trend for using 3D printers to turn all kinds of solid shapes into complex gear systems.