Sphericons

Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4877016.

Sphericons are mostly round so that they will roll, but unlike a ball, it will not roll in a straight line, it will follow the face of the sphericon with makes it, for instance, go straight for some distance and then go a bit to the side and then go straight again. The 7 and 9 are anti-Sphericons since it is an uneven number, these will roll forward and then backwards again.

They can be printed in two parts (front and back or left and right) and they can be connected with a core, make sure that you have the right core since there are three cores, one for the square, one for the hexagon and one for number 7 to 10.