Doctor Who Series 10: 13 Easter Eggs & References You Might've Missed In 'The Pilot'
5. A Big Box Inside A Small One
Of course, when Bill discovers the TARDIS' true nature she utters the immortal line: "it's bigger on the inside." The phrase has become synonymous with the TARDIS over the years - even before it was actually mentioned in the show. The first time the line is ever used occurs in 1973's The Three Doctors. "Aren't you going to say that it's bigger on the inside than it is on the outside?" the Third Doctor asks Sergeant Benton. "Everybody else does."
In The Pilot, Nardole attempts to explain to Bill just how the TARDIS works. "First, you have to imagine a very big box fitting inside a very small box," he says. This is a callback to 1977's The Robot of Death, in which the Fourth Doctor illustrates how the TARDIS is dimensionally-transcendental with the aid of one small box and a bigger one.