3D Planes

Above is a flat picture of four spitfires. It is actually an stereogram optical illusion. You can trick your brain into thinking that there are three 3D images of spitfires by diverging your eyes so that they are focused spitfires two different spitfires next to each other. If you have ever viewed a magic-eye book you will know how to do it. If you haven't then here's how to do it:

Put your nose on the screen in the middle of the picture (not yet, read the rest of the instructions first!)
Do not focus your eyes; let them gaze off into the distance (this is the hard bit because they want to focus on the monitor).
Slowly move your head back. Keep your eyes focused behind your screen. If they focus on the picture start again.
When your head is at the right distance, the out of focus planes will appear to move together and come in to focus, in 3D.

If you can not do it don't worry. It takes a lot of practice to make your eyes focus behind and not on the screen. It is harder the farther apart the planes are and, if they are farther apart than your eyes are, it is impossible. It will be easier if you increase your screen resolution so that the picture is smaller.

How does it Work?

In normal life your eyes see everything from a different angle to each other. Your brain combines the images into one and uses the differences between them to judge distance. However, in a picture, like a photograph, both eyes see the image from the same angle; so it appears flat. When you are seeing the 3D spitfires, your eyes are focusing on a different planes next to each other. But your brain thinks that your eyes are looking at the same spitfire but from different angles (that is why each plane is at a different angle). Your brain is therefore fooled into thinking you are looking at a 3D spitfire instead of two 2D ones.