Here's one for all you book/book store/library lovers. Terry Pratchett's definition of L-Space:
Libraries, nature of.
Even big collections of ordinary books distort space and time, as can readily be proved by anyone who has been around a really old-fashioned second-hand bookshop, one of those that has more staircases than storeys and those rows of shelves that end in little doors that are surely too small for a full sized human to enter.
The relevant equation is Knowledge = Power = Energy = Matter = Mass; a good bookshop is just a genteel Black Hole that knows how to read. Mass distorts space into polyfractal L-space, in which Everywhere is also Everywhere Else.
All libraries are connected in L-space by the bookwormholes created by the strong space-time distortions found in any large collection of books. Only a very few librarians learn the secret, and there are inflexible rules about making use of the fact - because it amounts to time travel.
The three rules of the Librarians of Time and Space are: (1) Silence; (2) Books must be returned no later than the last date shown, and (3) the nature of causality must not be interfered with.
For those of you who may be new to Pratchett, check out this nifty Reading Order Guide (thanks, Chris, for sending the updated version!). It shows you connections between the books and lays them out in groups according to characters.