Ball bearing

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

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

The Tenth Doctor thought the commonly seen Earth cupcakes with were "genius". He said "No other species in the galaxy has ever bothered to make them". Rose Tyler later handed him a cake adorned with ball bearings. (TV: 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 arranged that the conduit could only be turned on by thinking about sunflowers and ball bearings. (PROSE: The Slitheen Excursion)

Behind the scenes

 * In the short 1978 sketch Merry Christmas Doctor Who, K9 Mark II told The Doctor and Romana I that he wanted "enough ball bearing to last out the series" for Christmas.