Nothing is ever impossible. Rose learned that well enough travelling with the Doctor. Never mind the number of times he said something was impossible - the fact that these seemingly impossible things were always happening disproved that well enough.

And here it's happened again. The impossible. You can't travel between parallel worlds, it'll tear both apart... but another lesson Rose learned long ago is that there are loopholes upon loopholes, especially where the Doctor is concerned.

And here she is.

Rose is not the child she used to be. She is forty. She has a husband and a child, and Torchwood to run. Her heart still jumps when she sees the TARDIS, and the man standing beside it with his back to her. Even when she can't see his face, she can see he hasn't changed. He's still tall and skinny, with fluffy hair, hands jammed into the pockets of his pinstriped suit.

There are so many things she could say, or do, and she's played out this scene in her head so many times, so many ways... If she really stops to think about it, she'll just stand here, she'll never say anything at all.

So she walks up behind him, and he's as oblivious as ever. He's too tall for her to actually speak in his ear, but she gets close enough as she rises up on tiptoe and says simply, softly, "You still owe me ten quid."