When I showed a friend my Mobius Figure 8 Knot, the large one printed in strong white plastic, he asked if it could be made so that a marble could roll around the surface. After a few moments I realized that if I modified the cross section to be a disk with three circles cut out just right, they would make a groove in which a ball bearing could roll freely all round the curve. Thus was born this Groovy Mobius Figure 8 Knot design. After three printings, I finally got the exact size and position of the groove not too tight, so the ball can roll freely, but closed enough that the 1/4” diameter steel ball bearing, once inserted, will not fall out. I have some 1/4" diameter steel ball bearings and would be happy to send one in the mail to anyone who buys this piece. You may wish to insert several, and let them chase each other around and around. This makes a very interesting desk top item, fun to play with, and attractive to passing colleagues. Better than a giant paper clip!