Rattleback

This simple thing behaves very unusually if you spin it. After several rotations, it begins to swing, and then rotates in the opposite direction. More details - https://ru.wikipedia.org/wiki/%D0%9A%D0%B5%D0%BB%D1%8C%D1%82%D1%81%D0%BA%D0%B8%D0%B9_ %D0%BA%D0%B0%D0%BC%D0%B5%D0%BD%D1%8C
The holes are made for M8 nuts
Link to video - https://youtu.be/xiTWg5cpeUw