ACE: Oh, "No coloured" signs in 1963, "Pakis out" and petrol bombs in my time - now it's the future and it's ethnic cleansing. Nothing changes, does it?

DOCTOR: Forty years ago, Harper wouldn't even have been out of the ordinary. It does change, you know. Never completely, never all at once, but it does.

ACE: But when?

DOCTOR: Are you looking for the butterfly?

ACE: What the one who beats its wings and tips the balance, so the hurricane forms? There isn't one, is there?

DOCTOR: Not often. They just tell the butterflies that to keep them happy.

ACE: I should have known.

DOCTOR: No, mostly they break the butterfly on the wheel of time. But over the decades, the millions of butterflies, the weather still changes somehow. That's time. A million, multi-coloured pieces of time.

ACE: You could turn it all around you know, reach your hand in and just... well, change everything!

DOCTOR: Yes. But then I'm a silly old man, with far too much time in his hands already.

DOCTOR: This is ridiculous! How can you go from plenty of time to running out of time in no time at all?

DOCTOR: Paul Tanner, congratulations, you're today’s lucky winner. Ace, tell the man what he’s won.

ACE: A complete set of explanations from the Professor here. And you have no idea what a rare and precious thing that is.

DOCTOR: Do you want a dangerous fugitive staying in your flat?

PAUL: Of course not.

DOCTOR: Well then, don’t upset him and he'll be a nice fugitive staying in your flat.

DOCTOR: You might want to break the window first.

WALTER: Doctor!

DOCTOR: After all, if it just bounces off the glass, well, fire-bombing her petunias isn’t much of a statement.