SCARLETTE: Do you feel well?

DOCTOR [after some hesitation]: It’s all right. I think I’ve just worked something out.

SCARLETTE: Oh yes?

DOCTOR: I think I know how to change myself into pure energy. Which means, I suppose, that I could move as fast as light.

SCARLETTE [playfully]: Impressive. How would you do it?

DOCTOR: Well, it wouldn’t be easy. It’d need a lot of concentration. I’d have to slowly self-adapt my biology. The tricky part’s forcing my body to convert the cell formations without disturbing the overall balance. It’d take a lot of practice and meditation.

SCARLETTE: Oh? How much meditation?

DOCTOR: I’d say it should take me about. . . ohhh. . . three thousand years. Less, if I could find a way of stopping myself getting distracted.

SCARLETTE: Three thousand years. To become a creature of pure light.

DOCTOR: I’m trying to decide whether it’d be worth the bother.