Rule #9: Telling the Doctor that a calculator can do maths sums quicker than he can is just asking for trouble.
"Come on, test me. Give me some numbers. I can do multiplication like that." He snaps his fingers to emphasise. "Addition? Easy. Dividing numbers that don't divide to produce integers? Easy. Easy peasy lemon squeezy. Come on."
"Let me just enter the numbers into the calculator, Doctor," Rose sighs.
"What, giving it a head start because you think it won't win without one? Fine by me. Let's call it a handicap, shall we? Like in golf. Yeah, a handicap. Make it a really big one, Rose, it's going to need it."
Rose almost tears her hair out in frustration. "No, it's because a calculator doesn't think, it just does. I'll press the equals button as soon as I've given you the numbers and we'll see who can work it out quicker, okay? It's fair that way."
The Doctor nods, bouncing on the balls of his feet. Rose had offered him a seat, but he'd refused it, saying he gets so excited by maths that he finds it hard to stay still. She'd found it very hard not to yell the word 'geek' at him over and over again. She's still finding it hard to resist that particular urge.
"Okay," Rose begins, and the Doctor goes rigid. "Six hundred thousand, two hundred and fifty-six divided by seven hundred and thirteen."
She hits the enter button just as the Doctor yells, very loudly, "Eight hundred and forty one point eight seven three seven seven-"
"Sorry, Doctor," she interrupts, waving the calculator in his face, "Mr Calculator was quicker. And he didn't batter my ear drums as much."
"What?" the Doctor says, panic-stricken, as he grabs the calculator from her and looks at the answer. "I so got that before this thing! What are you-"
"Yeah, but it took time to yell it out, didn't it? The calculator just told me with its handy display. You don't have a handy display, do you? Just a large gob." She can't help smirking.
The smirk's not on her face two days later, however, when the TARDIS has created a smashed calculator room specifically so that the Doctor can put the ones he's thrown against the wall in frustration inside it.
And it only gets worse when Staples gives them a twenty percent discount on all new purchases because their visits are so frequent.