"Whatcha doing?" Rose asked, leaning over the Doctor's shoulder and studying the piece of paper he was scribbling on.
"Just some sums." The Doctor muttered back distractedly.
"Mmm. I can't seem to figure this one bit out...It doesn't seem to work unless you divide by an infinitesimal number..."
"Huh." Rose said, mildly puzzled. "Looks to me like you're trying to divide by zero."
"We used to make jokes about that kind of stuff. You know, 'Don't divide by zero unless you want to get sucked into a massive vortex...thing.'"
"Mmhm." The Doctor replied, completely absorbed in his sums.
"Are you listening? Hello? Look, Jack Harkness is making out with a Dalek!"
"Tell him I told him not to bring his dates home..."
Rose sighed and rolled her eyes. "Just don't divide by zero, okay?" She said as she departed, leaving the Doctor alone with his figures.
"Divide by zero..." He repeated under his breath, glancing over at the TARDIS console.
"You sod!" Rose screamed as she hung onto the wide open door of the TARDIS, attempting to not get sucked into a massive vortex-like...thing. "You divided by zero didn't you, you bloody tosser!