Here's yet another thing that small children have in common with cats and dogs: They'll spend extended periods of time staring at something that only they can see. Occasionally, and more worryingly, they'll even smile and laugh at the invisible space, as if they were somehow interacting with it. What are they looking at, and why is it so hard to attract their attention away?
There are some who say that children are aware of things that we can't see, and that when we grow up, we forget the phantom visions we had as children. There are others who say that it's normal for children to have imaginary friends when they're very young. It may very well be, but we wish there were some way to be sure that their imaginary friend really is imaginary, and not the specter of some long-dead ghost who's sharing your living space with you.