In Doctor Who why does the doctor regenerate into the eleventh doctor?

Answer:
The tenth doctor regenerated because he absorbed all the radiation from a radiation chamber to save Wilf's (Donna Noble's grandfather's) life. Wilf knocked four times, so the Doctor knew it was time for him to die (because in effect the tenth doctor dies and the eleventh doctor is born).

Episode: The End of Time (part 2)
Contributor: ICanHelp95
First answer by ICanHelp95. Last edit by Saints 1256. Contributor trust: 0 [recommend contributor recommended]. Question popularity: 1 [recommend question].