He actually had his mouth open, ready to answer her, but obliged Mary's request not to say.
The Doctor shook his head.
"It's not good, but it's not a paradox until there's an overlap. I don't like hearing it, and I'd wager the TARDIS wouldn't let you near her without throwing some kind of fit, but that's not a paradox. If your mum were to become pregnant with you, and you're still here? Problem. Anything else is just simply making sure not to change a fixed point in time. Example?"
He shifted in his seat. Leather creaked. And his eyes shone.
"If, let's say, your dad's meant to do something amazing and great and wonderful and you stop him doing it, problem. Alternate time line. Can't have that. Fixed points are a big deal and are not to be touched."
There were so, so many of them, too. It was difficult. Time might not be a straight line, but it sure had a lot of sodding points.