For years I traveled everywhere in London using the London Underground. This meant that I ended up with a very disconnected mental map of the city. It was a bit like a video game world, where the parts you visit are mapped out in 3D, but the individual mapped areas are connected by doors that take you through empty unmapped space.
Each time I accidentally ended up walking from one station to another I would connect two previously unconnected regions of knowledge, and the effect would be revelatory.
When we moved to our current house, we soon encountered an old squirrel who considered our back yard his territory. For the last six years he has visited regularly, gradually getting tamer and tamer. In 2006 he got mange, and we treated him. Eventually he decided it was OK to take peanuts from my hand. Throughout the years he remained Alpha Squirrel, seeing off all challengers.
But a few weeks ago, I noticed he had an injury to his left front paw; some sort of swelling on one of the fingers.