 TARDISes are characterised not just by their ability to travel in time, but also their dimensionally transcendent nature.A TARDIS' interior spaces exist in a different dimension from its exterior, allowing it to appear to be bigger on the inside.The Doctor states that transdimensional engineering was a key Time Lord discovery in "The Robots of Death".<ref></ref> In the Ninth and Tenth Doctors' episodes, the TARDIS has an organic look, and the Doctor states in "The Impossible Planet" <ref></ref> that TARDISes are grown, not made.