Another from 'The Day of the Doctor' and another from Steven Moffat, this one's a bit like the joke in 'Vampires of Venice' when the Doctor pulls a huge UV light from his pocket, belittling Rory's small candle-like flame. "Yours is bigger than mine," Rory says. "Let's not go there," advises the Doctor. In similar circumstances, Eleven extends his screwdriver to impress Ten. Ten extends his - something he doesn't do very often - in competition, and is disappointed to see it just come up short. "Compensating?" he says. "For what?" "Regeneration, it's a lottery." Of course it's a dirty joke, but it's slightly cleverer than the one in 'Vampires of Venice' (though no disrespect to Toby Whithouse, or the original joke). You see, when the Doctor's extending the screwdriver, he's competing against himself. Eleven knows exactly what will give him the upper hand. Ten assumes he can level, and is incredibly childish in his defeat. Eleven feels emasculated by Ten's taunt. Ten is then hilariously smug about why he lacks the tiny details he envies in he successor. There are so many levels to that joke, so much context, it's not a simple penis joke. But if you'd like to see one that is...
