RUBY: But, hold on, excuse me. Doctor, am I being thick here? How do you not know your own granddaughter? I mean, don't you recognise her?
DOCTOR: Er... The truth is, Time Lords can regenerate their faces like a disguise. So we look different, but we're still the same.
RUBY: Right. Okay. But what for?
DOCTOR: It can save us if we're dying, or sometimes we can hide ourselves away...
DOCTOR: ..with a new face.
DOCTOR: Does that sound mad?
RUBY: Just don't go changing yours, okay?
DOCTOR: Oh, no. I'm keeping this one for a long, long time.