ROMANA: Newton? Who's Newton?

DOCTOR: Old Isaac? Friend of mine on Earth. He discovered gravity. Well, I say he discovered gravity. I had to give him a bit of a prod.

ROMANA: What did you do?

DOCTOR: Climbed up a tree.

ROMANA: And?

DOCTOR: Dropped an apple on his head.

ROMANA: Ah, and so he discovered gravity.

DOCTOR: No, no, he told me to clear off out of his tree. I explained it to him afterwards at dinner.