Loop coaster

A hand-cranked roller coaster loop. Turn the crank behind the board at an even pace and the two-car train hangs over the top of the loop, then whips through the bottom. Two egg-shaped gears out in front, one red and one white, do all of it, and you can watch them turn. The four riders' arms swing free, so they fly up over their heads at the top and drop at the bottom.

No electronics, no screws, no glue. 17 printed parts that click and press together.