Want to amaze your child during teeth brushing time? Ask him to watch his own eyes move in the mirror. He’ll be amazed that he can’t do it. I was also amazed when I first discovered this phenomenon. The explanation is simple; you can only see your own eyes when the light is reflected off the mirror straight back into your pupils. When you move your eyes off this line you can’t see them. Move them back and you are looking directly into your eyes again. No matter how much you dart your eyes about, they look to you like they never moved. It’s a great way to sneak an explanation of light and optics into a bedtime routine.
Another little known fact about eye movements is that you can’t smoothly scan your eyes across a room. Unless you are tracking something your eyes will skip from one point to another.
The Geekdad classic, The New Way Things Work, has an excellent description of the optics of the eye.