A yo-yo was a round toy which was spun up and down on a string.

While at the Time Lord Academy, the First Doctor would skip classes to practice yo-yos and juggling. (PROSE: Match of the Day)

When the Second Doctor visited the Panjistri homeworld, he had a yo-yo in his pocket, something which was also present on his visit to Gandor. (PROSE: Timewyrm: Apocalypse, AUDIO: Lords of the Red Planet)

The Second Doctor gave Mindy Voir a yo-yo, which Jamie McCrimmon had won in an arcade game, in exchange for signing his copy of one of her albums. (AUDIO: Helicon Prime)

The Third Doctor, realising that the Time Lords had programmed his TARDIS to always return to Earth, compared himself to "some kind of a galactic yo-yo". (TV: The Claws of Axos) On Silurian Earth, a yo-yo was found in the pockets of the cloak on the Third Doctor's body. (PROSE: Blood Heat)

The Doctor's fourth persona was the one who most frequently kept a yo-yo on his person. Although he once used it to test local gravity, (TV: The Ark in Space) he apparently used it primarily for fun. His skill with the yo-yo appeared to increase over time. (TV: The Android Invasion, The Brain of Morbius)

Leela spun the Doctor's yo-yo on his instruction, not realising he intended it to be fun, but rather believing it was part of the TARDIS' "magic". (TV: The Robots of Death)

Romana II once used the Doctor's yo-yo to test local gravity, and performed a complex figure-eight with it. (PROSE: The Romance of Crime)

The Sixth Doctor took a yo-yo out of his pocket while looking for a piece of wire. (TV: The Two Doctors)

It took Ace two weeks and "tons" of peer pressure to learn to use a yo-yo. She later traded her yo-yo to some children on Heaven for a Hoothi spore they had found. (PROSE: Love and War)

At the end of his life, the Seventh Doctor had a gold yo-yo on his person. Chang Lee stole the yo-yo, along with the rest of the Doctor's possessions, from Walker General Hospital. He later returned the property to the newly-regenerated Eighth Doctor. (TV: Doctor Who)

While on the Moon, the Twelfth Doctor used a yo-yo to inspect irregularities in gravity and to save Courtney. (TV: Kill the Moon)

The Twelfth Doctor later tried to use a yo-yo to impress a Viking village into believing he was Odin, quoting Clarke's Law. According to Clara Oswald, this was something that the Doctor did a lot to get them out of tricky situations. (TV: The Girl Who Died)