How long has THIS been going on?

Yesterday afternoon, awakening from a refreshing nap on my living room couch, I opened my eyes to find a black cat staring back at me from across the room.  A cat very different than my own. 

The thought that this creature was comfortable enough to stroll in and help himself to a kitty snack while I was snoozing just a few feet away makes me wonder what sort of cat shenanigans go on in my absence. This much is clear; my theory that untrained cats could not possibly master the intricate mysteries of cat-door operation does not hold water, and it appears that the concept of a hinged flap is not a difficult one to grasp after all.

Even for a beast with a brain the size of a persimmon.