Balloon

A balloon was an inflatable bag filled with gas. Balloons had a wide variety of uses.

Transportation
Hot-air balloons were a type of aircraft which were raised into the air by a balloon. In the 17th century, the Eleventh Doctor escaped from the Tower of London in a hot-air balloon, which witnesses called a "magical sphere". (TV: The Impossible Astronaut)

In the 1940s, barrage balloons were used in the United Kingdom during World War II. (TV: The Empty Child)

Science
In the 1970s, Dr Tyler used balloons to carry his cosmic ray monitoring device into the atmosphere. (TV: The Three Doctors)

While on Storm Mine 4, Chub carried out experiments with weather balloons. (TV: The Robots of Death)

The Eleventh Doctor used two toy balloons to illustrate the difference between the main universe and the pocket universe in which Hila Tacorien was trapped. (TV: Hide)

Toys
In 1913, Daughter of Mine carried Lucy Cartwright's red balloon with her after taking over her body. (TV: Human Nature)

In 1914, Aphasia carried a red balloon with her. (PROSE: Human Nature)

As a child, Alistair Gordon Lethbridge-Stewart lost a red balloon. The Eighth Doctor later changed time to a minor extent so that he caught the balloon and returned it. (PROSE: The Shadows of Avalon)

Balloons were often used by clowns, such as Odd Bob the Clown, (TV: The Day of the Clown) Clara the Clown, (TV: The Celestial Toymaker) and Chuckles and Boris. (TV: Robot Gladiators)

A young Mikhail lost his balloon. The Eighth Doctor found him a new one. (PROSE: Reversal of Fortune)

As Mickey Smith claimed Rose Tyler told him, the Ninth Doctor once landed the TARDIS in a big yellow garden full of balloons. (TV: The Christmas Invasion)