On Christmas Day, television audiences watched as a spark of light flew into Matt Smith's unconvincingly-wrinkled mouth and a new part of Doctor Who lore was born. Having (apparently) exhausted all his regenerations, the long-standing question of what he (or, more accurately, the writers) would do once he hit the limit was finally answered. The Eleventh Doctor, moments before he boggled at Clara with brand new-old eyes, told her that the Time Lords had bestowed upon him a brand new regeneration cycle - foreshadowed with the Seal of the High Council that the Master had received in 'The Five Doctors' with a promise of the same reward. Despite this, we still have no idea how regeneration works and 50 years of continuity have hardly helped matters. Have the Time Lords just picked a bundle of new faces off the shelf and chucked them through the crack or did they give him the ability to continue regenerating naturally? In which case, does regeneration just throw a random assemblage of body parts into something vaguely humanoid or is the appearance of each incarnation influenced by the Doctor's DNA? Since the Time Lords are meant to be dead/frozen in time/in another Universe (depending on what time of day you ask Steven Moffat), could they really do something as complex as giving the Doctor a new set of regenerations? What if the best they could do was reset his body clock? What if a new regeneration cycle literally means a cycle - going round the houses again, as the Fourth Doctor described them - and the Doctor's ability to regenerate has been rebooted? That would mean that Peter Capaldi could be playing the First Doctor, younger in appearance but physically identical and with similar mannerisms. The character hasn't been reset; he's still the same man he was as Matt Smith, just with a different (albeit very familiar) face. Obviously this is just a theory, but this may explain a few things we've seen already. Let's look at the evidence...