This is a video of Youtuber Henry Segerman demonstrating the 'mathematical art' four-way tetrahedral rack gearset he designed and had 3-D printed by Shapeways (you can get your own HERE if you're interested). I thought it was pretty fascinating. I also thought it probably has zero functionality whatsoever besides being the key to perpetual motion and/or time travel. But what do I know? I'm just a man who got straight A's in the o' retikal fizziks and went on to teach it to the world's greatest minds. "Theoretical physics?" So I failed spelling, that doesn't mean anything.
Keep going for the video.
Thanks to D Shark, who's convinced this gearset is the key to economical, near-lightspeed spaceflight, which I believe.