Ever notice how in scary stories, animals can sense spirits that you (humans) can't?
Ever notice how the spirits are supposed to move incredibly fast? Scientists say dogs (and some other animals) can't watch TV because their eyes are more efficient than ours, and all they see on the TV is one picture or just static.
Basically, videos are made up of pictures running at a high rate of speed, right? It would be like if I sped a video up 100x, all you would see is static, right? Same with dogs. Now, what if spirits aren't transparent? They're completely visible, but they move too fast for us to see, that might be why animals see them and we can't. That would explain the shadows you sometimes see out of the corner of your eyes...
Or what your pet's staring at right now.
I guess we'll never know, will we?