Why is a mouse, when it spins?

When you see this post, quote from Doctor Who on your LJ.

There are worlds out there where the sky is burning, and the sea is asleep, and the rivers dream. People made of smoke and cities made of song. Somewhere there's danger, somewhere there's injustice, somewhere else the tea's getting cold. Come on, Ace; we've got work to do!

The Doctor's final words, in "Survival"