What are they looking at?

I know cats are sensitive and can see the smallest and fastest flying insects. But sometimes, there really isn't any insect flying about and you still see the quick turn of the head, big attentive eyes, perky ears, all focused on something.

That disturbs me especially when I'm home alone and they go do that. Wonder if there's something else they're looking at? Sometimes more than just staring, they go running in the direction they were looking at. I take that as them acting up and just wanna play chase or get some exercise since I often say "look, your tummy is about to touch the ground when you're standing".

Guess I'll never know what all those are about. Shrug it off and turn back to my TV, whadaya think?