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If you mean how did the second turn into the third, then he was punished by the time lords, they forced him to re-generate and banished him to earth.
If you mean the third into the forth, he was "wounded" by radiation while fighting the spider queen, after he returns to earth he collapses and is helped by a fellow time lord to regerate.
What else can I help you with?
What will happen after the 12th Doctor dies in Doctor Who?
He'll regenerate into the 13th Doctor.
What will happen when the 11h Doctor dies?
He will regenerate into the 12th Doctor.
Will the 11th Doctor die?
Eventually yes, he will regenerate into the twelfth Doctor.
How did the fourth Doctor regenerate?
The fourth Doctor fell off a bridge.
Why did romana regenerate in Doctor Who?
Romana was another Time Lord, like the Doctor.
In what episode did the eighth Doctor regenerate in?
It is unknown, the Eighth Doctor only appeared in a movie.
Who did Tom Baker regenerate into as Doctor Who?
Peter Davison.
When does the Tenth Doctor regenerate?
In the movie, The End Of Time.
What does The Doctor name his daughter on Doctor Who?
The Doctor names his daughter "Jenny." Jenny was created by an extraction of the 10th Doctor's DNA, from tissue in his hand. Jenny has the ability to regenerate just as the Timelord race. This means she has The Doctor's abilities.
Why does the Doctor regenerate?
Time Lords regenerate because they have a injury or something is wrong, it's their way of healing themselves.
Is the girl on Doctor Who day of the moon the doctor?
The girl in the end of Day of the Moon seems to be Amys daughter and she can regenerate since she was in the tardis.
Where did the Eleventh Doctor regenerate?
Trenzalore (his final resting place), in the town of Christmas
