I've just been on the thread where someone has a weird ghostly image in the window and it reminded me about a task I used to do with students on a training course I used to run at work (not because it is ghostly but thinking about perception and the brain).
Watch this video concentrate very hard on counting the number of ball passes and try not to give away the ending on the thread. Any hint about it ruins the effect. It's not the same video I used to use but it is very similar. Give it a try...