Romana: Newton? Who's Newton?
The Doctor: Old Isaac. Friend of mine on Earth. Discovered gravity. Well, I say he discovered gravity, I had to give him a bit of a prod.
Romana: What did you do?
The Doctor: Climbed up a tree.
Romana: And?
The Doctor: Dropped an apple on his head.
Romana: Ah, and so he discovered gravity?
The Doctor: No, no. He told me to clear off out of his tree. I explained it to him afterwards at dinner.