Well, there's one which… which one of my editors found particularly surprising in which you have what is called a Necker Cube which is a drawing of a cube, just 12 lines showing the edges of the cube, on a piece of paper so you see 12 lines and you look at it and you see a cube not straight head on but slightly to one side like this, and if you go on looking… looking at it for some part of a second… some part of a minute or something or other, it will flick over and you’ll see a different view of the cube, and then if you keep on looking it’ll flick back to the original one. But there’s nothing actually changing. What’s coming into your eyes is… there’s nothing on the page that’s changing, what’s changing is your interpretation of the three-dimensional picture that you’re seeing which shows very clearly that your brain… and it so happens that two interpretations, because the way the thing is drawn, are almost equally plausible, so the brain looks at one and then that fatigues it in some way and the other takes over in a way we don’t fully understand, and so on, and you see that one… you never see both at once. That shows that what you’re seeing is a constructive process of the brain, you see. And there are a number of other things along those lines. For example, seeing in depth is not only due to see your two eyes but it gets slightly two different views of the scene but if you close one eye you can see in depth especially if you move your head, you see, because there are a lot of other clues in the visual scene which your brain interprets as being depth. Remember there’s no depth, as such, on the image on your eyes.