Ball bearing

Ball bearings were a type of rolling-element bearing which used balls to maintain the separation between the moving parts of the bearing.

The Doctor "mocked" K9 by handing him a bag of sweets and asking if he would like a ball bearing. (DW: The Invasion of Time)

The Doctor thought the commonly seen Earth cupcakes with edible ball bearings were "genius", and said "no other species in the galaxy has ever bothered to make them". Rose Tyler later handed him a cake adorned with ball bearings. (DW: Fear Her)

When a police force were trying to uncover Mamps, Cosmo and Leeb Slitheen from their stone prisons with a want conduit, the Doctor stopped them and made it so that the conduit could only be turned on by thinking about sunflowers and ball bearings. (NSA: The Slitheen Excursion)