Yo-yo

A yo-yo was a round toy which shot up and down on a string. The Doctor sometimes kept one on his person. He largely used it in his second and fourth personas to test local gravity. (DW: The Krotons, The Ark in Space). However, his fourth self may have gotten so used to using it scientifically, that he actually began to simply find it fun to play with. Indeed, his skill with the yo-yo appeared to increase over time. (DW: The Brain of Morbius, The Android Invasion)