What Does The Kitty See?

Sometimes one of the kitties will go to a window, and then stare outside intently. I will creep up behind them and look over the top of their head in the direction they are looking, to see what attracts them so. I have never been able to figure out what they see. And when they are in this mode, they are staring intently, hyper vigilant. It is hard to imagine how animals see the world - my best guess would be that some almost imperceptible movement catches their eye. I will never know. Frustrating for someone as curious as I am, wondering what makes the world work. She is beautiful, though, isn't she?