Rattleback

thingiverse

To add amusement, a spinning toy rotates freely in one direction but stops, wobbles, and reverses when spun the opposite way. Two printable files are available as mirror images with distinct spin preferences. For optimum performance, use 100% infill during printing; afterward, sand them smoothly along their curved axis. Experiment with different surfaces to control the degree of reversal. These unique toys were requested for upload by Spacexula. Further information can be found on Wikipedia's rattleback page.