some type of ubiquitous translation tool is necessary to explain to the audience how people from other countries, time periods, and even other worlds, can understand each other and indeed appear to speak (mostly) flawless English." The TARDIS also has other special abilities: it can produce a large, invisible air bubble around its exterior that allows occupants to survive in an area that lacks oxygen as long as they are close to it and in one episode, it can create a bridge tunnel that occupants can use to cross over to out-of-reach areas such as another ship. The TARDIS is also shown to be strong enough to tow other ships and planets and can even withstand black holes. Conceptual history Exterior design When Doctor Who was being developed in 1963 the production staff discussed what the Doctor's time machine would look like. To keep the design within budget it was decided to make the outside resemble a police telephone box, a common piece of street furniture that had originally been designed in the 1920s by the Scottish architect Gilbert Mackenzie Trench. The idea for the police-box disguise came from a BBC staff writer, Anthony Coburn, who rewrote the programme's first episode from a draft by C. E. Webber. While there is no known precedent for this notion, a November 1960 episode of the popular radio comedy show Beyond our Ken included a sketch featuring a time machine described as "a long police box". The concept of a cloaking mechanism (later referred to as the "chameleon circuit") was devised to explain this. In the first episode, "An Unearthly Child" (1963), the TARDIS is first seen hidden in a London scrapyard in 1963, and after travelling back in time ("The Cave of Skulls") to the Paleolithic Era, the police box exterior persists. In a subsequent story, "The Time Meddler" (1965), the First Doctor explains that the TARDIS should automatically adopt a disguise, such as a howdah (a carrier on the back of an Indian elephant in the Indian Mutiny) or a rock on a beach. Accounts differ as to the origin of the police box prop. While the BBC asserts that it was constructed specially for Doctor Who, it has been claimed that the box was a reused prop from the BBC television police dramas Z-Cars or Dixon of Dock Green (a claim later repeated by Doctor Who producer Steven Moffat). The