And as we all know, spare keys are quite hard to come by. You can't just check under the rock by the entrance, after all.

[It doesn't work, the not causing mischief bit. And even if you lock people inside, they always manage to find a way to wander off. Always with the wandering off.]

What's he like, this Doctor? Just out of curiosity.

[He doesn't really know her. Not really. So, one of the later ones, then. Assuming he has later ones. But, well, it's always a good idea to keep up to date with yourself.]