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.