Every few series or so, the Doctor regenerates. When he is in critical condition, he becomes a different person (only visually, his personality doesn't change all that much) and thus a new actor is shown. (e.g. Christopher Eccleston "regenerates" into David Tennant.)
The real reason behind this is because the actor's contract with the show ended, whether by choice or by force.
At the end of the Fifth Doctor story "The Caves of Androzani", The Doctor and his companion Peri are exposed to a lethal dose of toxic 'Spectrox'. With only one dose of antidote, the Doctor saved Peri, and then regenerated into the Sixth Doctor, played by Tom Baker.
He used Regeneration, a Time Lord skill that allows the Doctor(or any Time Lord for that matter) to live on. The new Doctor will have a different look and personality but will retain the memories and abilities from the previous Doctors. In the Sixth Doctor's case, the TARDIS crash landed after being attacked by the Rani. He was injured enough in the crash that he regenerated into the Seventh Doctor.
the 9th doctor (Christopher Eccleston)
Robin Williams
yes he does
Robin Williams
Not during this regeneration, but that would be hilarious if it did!
Probably turn of the century.
saratoga? Energy is what caused the tide to turn in favor of the Americans. This is taught in History.
Your left foot turning blue would be caused by cyanosis. This is when the blood does not get enough oxygen in it. You should immediately contact a doctor regarding this.
The episode after "Turn Left" is "The Stolen Earth"!
Matt Smith
No lol