I couldn't help glancing into this before heading to the office this morning. 20p later I reluctantly dragged myself away. It's already provided a neat insight into co-ordinate systems.
Topology joke my brother used to tell:
Q: How can you escape any prison cell, only using mathematics?
A: Simple! Perform a co-ordinate transform such that the outside of the cell becomes the inside and vice versa - you are now free!