Page "TARDIS" Paragraph 23
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Apart from the ability to travel in space and time ( and, on occasion, to other dimensions ), the most remarkable characteristic of a TARDIS is that its interior is much larger than it appears from the outside.
The explanation is that a TARDIS is " dimensionally transcendental ", meaning that its exterior and interior exist in separate dimensions.
In " The Robots of Death " ( 1977 ), the Fourth Doctor tried to explain this to his companion Leela, using the analogy of how a larger cube can appear to be able to fit inside a smaller one if the larger cube is farther away, yet immediately accessible at the same time ( see Tesseract ).
To those unfamiliar with this aspect of a TARDIS, stepping inside the ship for the first time usually results in a reaction of shocked disbelief as they see the interior dimensions (" It's bigger on the inside!
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