Martha: But are we safe? I mean, can we move around and stuff?
The Doctor: 'Course we can, why do you ask?
Martha: It's like in the films! You step on a butterfly, you change the future of the human race!
The Doctor: Tell you what then, don't... step on any butterflies. What've butterflies ever done to you?
Martha: What if... I dunno, what if I kill my Grandfather?
The Doctor: Are you planning to?
The Doctor: Well then.