I do not own Doctor Who :(
The Doctor and River were in the Console room, flying the TARDIS back from October Thirty-first, 1341; The first Halloween. They had gotten into a conversation about the traits and genetics of Time Lords, which had brought up a very interesting subject…
"I bet you can't." River said smugly, flipping a lever and crossing the glass floor.
The Doctor crossed his arms.
"Alright, I'll admit I've never done it before, but that doesn't mean I can't!" He protested. "I knew a Time Lord who did it just to suit his mood. Or her mood. It really got quite confusing after a while..."
River shook her head, blonde curls bouncing.
"Nope. That's a lousy excuse, Doctor."
"Oh yeah, wanna bet?" He asked confidently, flipping the stabilizers and locking eyes with her.
"I bet you I can, I'll do it. But if I do, you owe me."
"How much?"
"Twenty Quid."
A/N: Disclaimer: I have no clue when the first Halloween really is, but this is fiction. If you wanted facts, you should've read an encyclopedia. Oh, and the Gender-swapping Time Lord the Doctor is referring to is The Corsair, who is mentioned in the episode The Doctor's Wife.