NARVIN: Look at the whiteboard! Was that you who wrote that?

DOCTOR: Oh, that, yes, sorry... I noticed you'd made a couple of mistakes, sorry, I was trying to be helpful. You see, if x is defined as—

NARVIN: Helpful? I spent hours working through those equations.

DOCTOR: Probably why you got them wrong, I'm sure you were tired.

NARVIN: They weren't wrong! And what's that? Bottom left.

DOCTOR: Oh, that's just a risotto recipe that occurred to me while I was correcting your maths. I jotted it down so I

didn't forget it. Then I forgot it, sorry. Do you have a spare piece of paper? I could just copy it down.