We have 2 indoor-only cats. Our neighborhood is quite full of roving outdoor cats, and our backyard is a cat freeway. There are at least 6 cats that come by, just passing through, several times a day.
As you can imagine, this drives our cats NUTS. They chatter to them through the glass, they smack the windows, they run around the house getting other angles. It is this feature that amuses me most. There are 2 windows onto the yard that the cats can see through, one in our bedroom, and one in the living room. In between the two there is a fireplace. Every time Misha (the more territorial one) is in a state of high alert because she has Seen The Enemy, she does a cycle of living room windows, fireplace, bedroom windows, and back. I’m not sure what it is that makes her look at the fireplace – the fact that it’s glass, perhaps, or that it’s such dark glass that she sees herself in it and it sets off the kitty detection thresholds. In any case, it’s highly amusing.