Why do cats stare out the window?

United States
July 13, 2008 3:44pm CST
Right now, my big cat is sitting on my computer desk and staring off into the distance out the window. What's he looking at? I can't see anything, but he seems very upset by something outside the window. His eyes are small and his mouth is very tight as if he's scared. Maybe he's just contemplating the sky! Have you ever seen your cat stare out the window at some imaginary thing? What are cats looking at when they stare out the window? Is there something out there, I can't see?