refers to when his original incarnation was a "kid" at 90 years old. However, within a specific incarnation, a Time Lord is able to age, albeit much more slowly than a human. The War Doctor and Eleventh Doctor, over the course of the Last Great Time War and the Battle of Trenzalore, respectively, are seen to age within their respective incarnations to what would appear to a human to be old age; both conflicts are suggested within the series to last hundreds if not thousands of Earth years. In The Daleks' Master Plan, the First Doctor is able to resist the effects of the Time Destructor better than his companions, who are visibly aged by it; one of them, Sara Kingdom, ages to dust before the Destructor device can be reversed. The Fourth Doctor is briefly aged 500 years in The Leisure Hive, leaving him an old man but still active. A similar situation occurred in "The Sound of Drums", where the Master uses specially made technology to age the Tenth Doctor by a century, leaving him in a frail and helpless state. A further application of this technology in the following episode, "Last of the Time Lords", suspends the Doctor's capacity to regenerate, showing the effects of 900 years of life without regeneration. In The Two Doctors, the Second Doctor states that the "Rassilon Imprimatur" allows Time Lords to safely travel through time, becoming symbionts with their TARDISes, and that the reason other species are incapable of developing time travel is that they lack the imprimatur. However, he implies later that he was lying about at least some of this information to mislead the Sontarans. At the beginning of The Trial of a Time Lord, the Sixth Doctor suggests that a number of elder Time Lords were able to use their combined mental energy to summon his TARDIS against his will. Time Lords can survive, but not function properly, without two hearts. In "The Shakespeare Code", the Tenth Doctor has only one heart working. He knows this and tries and fails to stand up, until both of them start working again. Also, in "The Power of Three", after a massive electric current is passed through the Eleventh Doctor, he is left with only one heart in working condition and is unable to carry on for a long period of time. In "The Unicorn and the Wasp", the Tenth Doctor