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.