If you ever see that Doctor again, thump him on the head for me. This place is a temporal nightmare, of course there'd be two of us eventually.

[ but yes, back to the question. ] When a Time Lord is near death, they can choose to regenerate into a new body. New outside, same inside. New appearance and personality, same memories. I'm... [ well there's a pause here because does he start counting again because he got a new set of regenerations from the Time Lords? ] I'm Doctor Cross Eyebrows, he's Doctor No Eyebrows. Both still the Doctor.