[All right, so we're talking someone way out of touch with technology. At least that gives him a better idea of just how pedantic he's going to have to be with all this.

As he, y'know, talks with some wry humor to a completely black screen.]

And now that you've covered it, I can't see you anymore. Think of it as something like a window or a door. The window is the screen — letting you look at whatever is on the other side — and the camera is the "eye" of whoever is looking back at you. Right now, you're covering my eye, so I can't see you. But the eye on my device is still "open", which is why you can still see me.

[He waves, slightly, for emphasis.]

However, if I were to close your "eye", the way you're doing to me now...

[As he puts his own thumb over the camera.]

You can't see me anymore, can you?