Have you ever seen a random dot stereogram? You see them on posters, cards, calendars, etc? It can be a random pattern of dots or a repeated pattern of just about anything, and if you stare at it in just the right way you suddenly see a 3 dimensional image sticking out of the page. Have you ever wondered how they are made? Lets take a look.

- You will need your computer and a program where you can type.

Begin with a repeating pattern. Lets use the numbers 0-9. We repeat the pattern over and over.

0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789

When you look at the pattern, you want your focus to be behind the page, so that your right eye is seeing the pattern in one place and your left eye is seeing the same pattern in a different place. You can try this with the pattern above, but it won't do any good. Both eyes see the same thing, so it just looks flat.

Now, in the same block pattern, in the 4th, 5th, and 6th rows, we will leave out a 0 in the middle of the row.

0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789
012345678901234567890123456789123456789012345678901234567890123456789
012345678901234567890123456789123456789012345678901234567890123456789
012345678901234567890123456789123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789
0123456789012345678901234567890123456789012345678901234567890123456789

Now look at the pattern in the same way you look at other stereograms. When you get your eyes focused properly, you will see a block of numbers raised up in the center of the three lines that we removed the zeros. Why does it look that way?

To understand that, we need to understand how we see things three dimensionally. Look at something in the room. It could be a lamp, a chair, or even your little brother, if you can get him to sit still long enough. Now, close your left eye. Look carefully at the edges, noticing which parts you can see. Then switch eyes, closing your right eye and opening your left. Do not move your head. Notice that the image you see has changed a bit. Since it is looking from a different angle, it can see some parts that your left eye could not. Also, some parts that your left eye could see are hidden. Your brain combines these two different images into one three dimensional image, letting you see depth by comparing the two images.

With the top stereogram, both eyes are seeing different parts of the pattern, but because they are both the same, you still see a flat image. In the second stereogram, we changed part of the pattern. When you look past the page, your eyes focus on different parts of the pattern. This time there is a difference; some of the zeros are missing. Your brain decides that they must be behind an edge, so you see the block of text sticking out towards you.

Once you can see this one, get out your word processor and try adding and subtracting characters to see how it changes the 3D image.

If you have trouble seeing the image, there are several things you can try. One way is to stare at something that is about a foot behind the page (or your computer screen) and then quickly look at the pattern without focusing on it. Another is to place two pennies on the table and look at them, letting your eyes go out of focus until you can see three pennies. Some people have more luck with this than others.